So lets say that you’ve traveled back in time with no hope of return and have spent the last few years using your knowledge of the future to secure yourself the throne of England. You have killed or suppressed all of your rivals so that at current moment you’re the unquestioned leader of England.
So the question is what is your next step?
Start instituting a bunch of public health reforms – clean water, separating sewer systems, physicians washing hands, etc. Abolish “bleeding” as a health measure (perhaps wait for other reforms to start having positive effects to justify it). Just knowing germ theory should make a huge difference in general mortality.
Also, institute the assembly line for production of various goods – I think even with low tech, assembly line production greatly increases efficiency. Utilize capitalism for the public benefit – provide monetary incentives to inventors and perhaps something like patents, such that inventors can massively benefit from their ingenuity (i.e. they get a penny for every newly-designed plow sold).
Slowly but surely increase the rights of women. Employ some smart and charismatic women in court.
Invest in public education and literacy. Invent the printing press. Print an abridged bible that emphasizes the golden-rule stuff and make it widely available.
Write down everything I can think of that might help future generations, emphasizing the value of human rights. Sow the seeds of democracy.
Treat the Jews well.
Watch my back against the damn nobility. They’ll probably be pissed.
You would have to affirm the Magna Carta so you couldnt be an absolute monarch and your ability to communicate with others would be limited. Your immune system being not used to the local bugs and would either kill or severly sicken you. You would have to deal with the French and the Scots…
You would quickly learn why soap, deodorant and toothpaste are important. You would come to fully understand the meaning of the word boredom as there is little to do but eat and have sex…ergo…
I would tap my heals together three times and whisper…there is no place like home…there is no place like home…
If you had enough mana to tap your heals three times you wouldn’t fall prey to the local bugs 
Sewers, and roads at home. At first, roads will be between the cities, likely upgrading and replacing the old Roman roads. Not just cobbles but tarmac. I will encourage this by selling monopolies - if you want to use the road, you pay the owner. Instead of being executed, common felons will serve time breaking stones, mining gravel, building roads (for which the owners will be charged), bridges, and other civil works before being granted their freedom.
I will encourage the foundation of schools. I will institute an academy of sciences. Charlatans will be dealt with severely. Generally, I will try to institute organisations that will continue after my death.
I will conquer Wales then Scotland will keep the nobles busy. Wallace and the Bruce will be killed. Quietly, not publicly. I am still king of large parts of France. I will secure my holdings there.
It’s too early for modern-style patents - the legal system isn’t yet up to it and too few people can read and write. Remember also that travel is difficult. Rather, when on Royal Progresses, I will immediately reward inventions and encourage my nobles to do the same. 1300 is about the time the Livery Guilds are formed in London, so I would encourage those. Again, it’s too early for limited companies, for much the same reasons as patents.
The Black Death is in the offing - likely beyond my lifetime, but I will want to prepare my country for it.
Work on building Rule of Law as the primary mode of government. Establish an administrative bureacracy. Regularize, rationalize, and codify revenue streams. Build a standing professional core to the army. THEN bring the nobility and church under hand. THEN build universities, and fund them. Then enourage the universities to work on things like health, sanitation, medicine, etc. (with a few prompts from future knowledge)
I figure I’m going to get assasinated somewhere in the middle of the taxation step. 
I’ve apparently just murdered Edward I and his heirs. This might not be totally unpopular, because Edward had gotten involved in a series of expensive and pointless wars with the Scots in an attempt to claim overlordship and thus make himself de facto ruler of British archipelago. It turned out to be an overreach.
So… diplomatically abandon all claims to overlordship, leaving the Scots to fight amongst themselves over who should be king. Defend the North of England if need be (either from opportunistic raiding or from showboating by title claimants) but end attempts to impose political will on Scotland.
Start on the succession. Presumably through a strategic marriage to a powerful noble family (you can never get rid of all your rivals to the throne, because thrones generate rivals simply by virtue of existing). But without an heir you are practically begging to be usurped, and rightly so. Monarchy is only as a stable as the succession. It might be politic at this point to be somewhat indiscriminate in one’s reproductive strategy, because you can always recognise a bastard as heir if need be.
Edward I had subdued the Welsh by this point, but they might feel a new reign is an opportunity to push back. For reasons of prestige any attempt at that is going to have to be slapped down sharpish. Again, I don’t want to be involved in years of tax-reliant warfare, so the only viable military approach will be to go hard early. I’d rather not do that, given my namby-pamby 21st century review of mass slaughter and punitive rape as war crimes, so probably I will try to buy off and/or threaten potential ring-leaders early on.
Edward’s last attempt to make war on France had failed, so there’s no need to stir up antagonism. I won’t make a big deal about getting Gascony back - any claim would be very tenuous.
That leave’s iiandyiiii’s programme of public health, and probably something around education. Public health is important, because the Black Death is due in about 40 years. The Crusades have opened up contact with the Middle East, so a sponsored programme of scholarship might kick start a renaissance type event in England some years early.
There are no Jews in England at this point - Edward I expelled them 10 years ago. Invite them back, partly because well come on now, and partly because access to finance markets supports the development of trade. On that note, if all goes well and I’m still in the job 20 years from now, I might start sponsoring voyages down the west coast of Africa and encourage my captains to consider the wisdom of being blown a couple of thousand miles off course from time to time on the grounds that you never know what you might find.
Actually, now is the time to conquer Scotland as it has yet to define itself as Scotland.
Hmm thinking of the black death it occurs to me that if you ensure the invention of antibiotics but make it a crime punishable by death if they are shared with foreign doctors you may be able to conquer Europe without too much trouble.
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Without modern communications, there is absolutely no way you’d keep such knowledge contained. People generally didn’t consider information the same way we do today, and you’re not going to easily change it. Some doctor is going to decide to set up shop in Europe and make a mint for themselves - and you’ll never know about it until they’re long gone. Actually, many doctors will do so.
On the other hand, Europe will still depopulate pretty effectively - you haven’t seen for conservative like folks were back then. “Modern” and “new” were NOT selling points, unless you could show the immediate advantages. And even then, most ideas would face significant resistance.
Since I would be Queen rather than King, my first order of business would be a strategic marriage. I would look for the medieval equivalent of a nerd - intelligent enough to father worthy heirs, yet uninterested in political power and enlightened enough to let his wife be the ruler. Ideally he would come from a powerful family willing to use their muscle and influence to back up my reign for now, with the understanding that a member of their family (my heir) would one day have the throne.
I would institute sweeping health reforms and encourage cleanliness in both person and environment. Public Health bureaucracy would be funded and favored.
Agricultural reform would also be implemented. A well-nourished, healthy kingdom of subjects would give me a tremendous advantage when putting together an army for the inevitable wars I would face. Agricultural advances would also lead to more surpluses - more crops to sell, more revenue to tax.
I would likely encourage a war of foreign aggression to give my nobles something to do other than plot my overthrow.
My nerdy consort could be given the task of sponsoring scholarship, education, and the development of libraries.
I’d have some sort of huge ball/party where I would sing I’m Henery the Eighth, I Am and possibly She Loves You (yeah, yeah, yeah). I’d also release a bunch of crazy prophesies that would one day prove true, like, "Two men, named Neil Armstrong and Edwin “Buzz” Aldrin, from a country that does not yet exist, will walk upon the moon.
In short, I’d fuck with people.
And put up with hunting what would be effectively guerrilla forces in the Highlands without a decent road network to move logistical supplies into the region? He’s already going to have to deal with the Welsh, why complicate his adventures?
Perhaps you should draw north and south America on a map and tell people to go find it.
It happened to me. I killed William Wallace, established a permanent parliament, reformed the law, and became known as the English Justinian. I fucked up when I expelled the Jews, I just figured they’d be happier someplace with a better climate.
You wouldn’t go the Elizabeth I route? Don’t forget that childbirth was very risky, even for the highborn.
Not really. You destroy the crops and villages and the bandits starve and freeze over the winter. Then you kill the rest the next spring / summer.
Except I thought the point of conquering Scotland is to put those productive lands under your rule. Killing and burning and destroying everything helps in what way exactly?
Killing and destroying everything is what you do what you know you can’t conquer a territory and you’re just trying to deny the revenue of that territory to your enemy.
But if you’re backing off on the Scots for a generation then there’s no point in massacring the peasants and destroying their homes and farms.
In the feudal era all wealth is derived from productive lands and the peasants to work those lands. So sure you want to stop the bandits. Because you want the wealth generated by the peasants without having to worry about bandits. Killing the peasants means you get nothing, you LOSE, GOOD DAY SIR.
Of course this is what the English used to do in France. But that’s because the English kings knew they couldn’t hold and conquer that territory, so you burn and loot and murder to deny the revenues of those lands to the French, and to demand concessions to stop.
Of course this is all silly. You can’t just become King of England just by defeating the previous King.
You’re only King when the other aristocrats agree that you’re king. So how exactly are you convincing them to do that? What’s in it for them?
The Medieval period is a long story of kings trying to centralize power, but the nobility clawing back that power because the monarchy needed the support of the nobility. France at this time is barely a kingdom and more of a confederation because French nobility are more powerful than their nominal liege lord, the King of France. Not to mention the problem that one of the French monarch’s vassals somehow got himself made King of a whole nother country. And it isn’t until the 1600s that the balance tips decisively in favor of the central governments all over Europe, and it isn’t until the 1800s that this process of centralization is complete. Italy and Germany aren’t unifed even in name until 1871, for crying out loud.
You only kill and burn and destroy those who oppose you. Then you move your own folks in. Land is a great incentive for service and there are plenty of younger sons of nobility seeking their own titles.
Send ships west. Claim it ALL!
Drink lots of Ale, consort with bawdy wenches by the score, eat turkey legs.
If I have advance warning that I’m being sucked back in time, I’l study the following:
-How to manufacture antibiotics (and if it’s even possible with 14th century technology).
-Same with quinine.
-Same with an antiseptic formula.
-Basic principles of medical care, including how to perform surgery and have safer childbirth with the primitive tools available.
-Food production technologies.
-Who are contemporaries I’ll be able to trust drawn from intellectuals/scientists and Christian figures (see below).
-The basics of offset printing and what tech might exist to put together an early press (so England can have the tech 140 years before Gutenberg and emerge as a literacy powerhouse).
-In-depth study of history (see below), including land and sea cartography.
When I get to the 1300s, as Queen Jennshark I will gather my group of trusted intellectuals and Christian figures. I’ll tell this group something along the lines of “I have revelations from God that he said you are to carry out”; this, hopefully, would make me safe from being accused of sorcery when it’s respected cleric who deliver messages. These bearers of knowledge will produce and introduce antibiotics, quinine, and antiseptics. They’ll also attempt a cultural change in how medieval folk regard regular bathing and clothes cleaning; much of the population believe that bathing is bad for the body – or even against God’s wishes – so these new customs will probably proceed very slowly. Same processes with boiling/filtering water and the need to improve general cleanliness (especially bathroom customs).
Using my in-depth history studies, I would try to change:
-How the black plague is treated and how to lessen the chance of catching it; interestingly, some scientists today speculate that the plague, or at least some epidemics labelled as such, was actually weird epidemics of disease(s) that have disappeared. Hopefully I can figure out the causes if it’s not rat-fleaborne. New food technologies could ensure the working/feudal classes are better fed and therefore more disease resistant. At the least I’ll manage to warn about the plague-carrying ship that will come into the Genoa port and spark and epidemic.
-The Spanish conquest of the New World. Since this is a future event, perhaps I could have one of my respected seers make a prophecy that the expedition is forbidden by God (and you’ll bring syphilis back to Europe! Your nose will fall off!) If it seems the Spanish will ignore a warning from the 1300s, I’ll dispatch a ship full of arquebuses, horses, and other technologies of war to the Maya and Aztec empires – this is where my cartography comes in. My sailors will frame the arrival of the Spanish as a warning from their gods and prep the population for resistance if the Spanish plow ahead with their plans in a century or two.
-Issue a warning about evil future monarchs. I probably can’t change history, but the world will be better if Henry VIII doesn’t go bananas and Queen Mary is prevented from creating bloody havoc. This, again, will come from my “oracular” group. I’d go all Nostradamus and write a book of prophecies aimed at preventing future genocides and holocausts. Finding a safe repository where the manuscript will survive will be a tough one.
-I’d order that a Domesday-like census is done every five years and figure out a safe repository for this knowledge so future folks have it.
-Religious tolerance. This is a tall order! I’ll give it a shot and have my trusted clerics preach that it’s sinful to repress and kill the Jews and burn heretics at the stake. This is probably too culturally-bound to make a lot of difference, but it’s worth the effort.
And I’d do a totally selfish thing! I’ll gather all the money, jewels, and other hardy pieces of wealth and bury it in a place that my 20th century ancestors can find and cash in on. I’ll use my history knowledge to find a safe place; perhaps this will be under a church that will still be standing and undisturbed by archaeologists. I’m not sure how to get this knowledge to my ancestors, as it would involve having people pass down secret knowledge for 700 years (and not act on it for their own enrichment. I’ll have to think about this a lot.
In addition to these efforts and actions, I’d have fun! I’ll travel all over, introduce new musical and entertainment genres, and change women’s fashions to be more masculine and practical (I refuse to go back in time if I can’t wear breeches!)