London’s sprawl necessitated means other than walking. By the time most cities got that big, trains, bicycles, automobiles and trollies (drawn by horses, but with a scale of efficiency) minimized the need for personal draft animals.
“Canning” – preserving food other than drying or salting – was invented by the French during the Napoleonic era, say, circa 1800. A way was needed to feed a large army that was in an area where they could not forage for any of a number of reasons.
They used champagne bottles because they were sturdy, filled them, corked them, and boiled them to kill the pathogens. They had no idea why it worked – Pasteur was not even born until 1822 – but it did so they went with it.
A lighter. I personally like the feel and touch of the Bic™, but i guess a Zippo would be feasible even in Roman times, as it has no plastic, if one finds the cerium for the sparks.
Yeah, the Appert canning method could have been done with 1725 technology, it just wasn’t. That being said Appert started work on it in 1795 and it took him 14 years to come up with it, and he specifically started his work because of a prize competition put out by the French state, so it was both non-trivial to develop and also seemed to require a bit of requisite reward promise for someone to take time to do it.
The method Appert ended up with likely would have been useful to people for home canning, as it was very labor and time intensive, but it had issues being scaled up to industrial level (which was what the French were ultimately hoping for, as a means of mass producing shelf-stable food for their armies.) But based on Appert’s work some guys in Britain developed the tin-canning method not too many years later which was still expensive, but a little more suited to mass production.
They actually had a process called “potting” that wasn’t nearly as effective as canning but already worked on some of the same principles. Basically you took meat freshly cooked, while it is still hot, and packed it into a jar, then poured melted fat on top of it (beef tallow commonly used.) When it cools the fat basically seals the food in underneath it, making it less likely for bacteria to grow as fast as meat simply left out. However, this doesn’t create a perfect vacuum seal, and is not shelf-stable for all that long.
There are many accounts of potted meat “keeping” for 4-6 weeks though. Because of the imperfectness of the process, sometimes you’d go to retrieve the potted meat and it would be spoiled, so it was by no means perfect. Often times one of the use cases for it was when you had meat you couldn’t finish off in one meal but wanted to preserve, so it was essentially a way to try and avoid waste, some incidental spoilage just being a cost of doing business.
Edit to add: I wanted to mention I first heard about potted meat from the excellent Townsend’s YouTube channel, which is a great source of information about 18th century daily life:
I think some of you are putting the cart before the horse: You can’t take anything back to the 1700’s and expect them to be able to create many of them, because the concept of interchangeable parts didn’t exist, and the tools of the time weren’t accurate enough to make them. Almost everything, down to screws and nuts, were laboriously handmade and bespoke. A screw made for one copy of a thing wouldn’t fit in another, etc.
Without interchangeable parts, you can’t have assembly lines. So any tech you take back, no matter how simple, will not be available to the masses.
So, I’d take back a bunch of sets of engineering standard weights and measures, and distribute them among countries that have the capability to build stuff, so that they could build and sell parts between each other, to help facilitate global trade.
Modern devices and products require a very long chain of technology to create them. Something as simple as lacking a perfectly flat working surface can prevent needed precision for manifacturing modern goods. Early factories used water wheels to power belts that the power equipment attached to. This resulted in varying speed and other issues that played havoc with precision. So we needed steam power and regulated speeds, which in turn required…and so it goes.
Perhaps look for some area where, for whatever reason the technology already existed do something but it wasn’t invented for some time for other reasons.
If you wanted to make the biggest lasting change, a good candidate would be to go back to the early Roman era with a box full of compasses and maps of the world printed on something durable.
Good luck manufacturing a gas-tight metal/glass container and creating a suitable flammable, volatile liquid.
That’s actually something John Townsends points out in several of his videos where he’s building a cabin using period tools/equipment (specifically the sort of cabin a family might throw up their first year on a homestead in the 18th century–they usually tore these down in a year or two and built a more proper dwelling), any type of fastener was expensive. A structure like that simple cabin might be made with only a few, or no, nails. If you used any nails, you’d prioritize it for very specific joints where they were going to really help. Additionally, when the cabin came down (which they usually did–this is one reason so few “settler cabins” survived), you didn’t just casually take it down and burn all the materials in a bonfire. All of the materials were valuable–including the nails, you would absolutely go through and make sure you recovered every single nail in there. The nails of the time were bulky and crude, and could easily be hammered back into fine shape if they had been bent or etc.
Zippos did not use gas when I still had them, it was a liquid. Much easier to contain with some cotton. And a flammable liquid in the quantities required for lighters? Petroleum is a Latin word, distilling is an art with many uses.
But of course I have not thought this through: what is the use of a lighter without cigarettes? 
There was nothing to firemaking in the Roman era or in the wider distant past. People have always needed fire at will, so have invented a wide variety of methods to that end for several hundred thousand years.
I can twirl two sticks between my palms and make a roaring fire in under a minute. No need for even string. And I’m only a hobbyist.
Toothbrushes go way back, probably to prehistoric times. You can buy Miswak/M’swaki/Siwak sticks on the internet, if you really want to
Of course, it wasn’t used everywhere. You could do a lot of good by introducing tooth brushing where it wasn’t regularly practiced.
I am very skeptical of your claim about twirling two sticks in between your palms and having a roaring fire in under a minute. Can you link to video or show any proof of anyone using this technique? I’m not an expert on primitive fire making techniques, but I know people who are, and all of them have always said the same thing to me–that all primitive fire making techniques are labor and time intensive. Les Stroud specifically mentions he’d never go into the wild without an easy fire starter tool of some kind, ditto for keeping one in his car, and he’s literally made a career on doing things like primitive fire making on video etc; but he says the reason he’d try to avoid having to do that is because primitive fire making techniques are all time and physical energy intensive, so he would rather be able to spend his time focusing on shelter and other pressing needs in the heat of the moment vs spending energy and time getting a fire going, if he could avoid having to do so.
Every household already had oil lamps, and fire strikers (flint + iron pyrite) to light them with. 
Vaccination using cowpox does not require any advanced technology. And it is just possible that eating a certain kind of bread mold might cure some infections.
The moveable type printing press to just pre-Imperial Rome. Like, Caesars’s still just a successful general-era Rome. Just for shits and giggles.
But what specific discovery are you bringing back?
Except for the doornails.
That’s an interesting video, at least some of those nails don’t appear to be used to fasten anything, is there actually a purpose to those nails or are they purely decorative?
Are there maybe two layers of planks in that door - with the layers staggered, so that any given nail is near the left edge of one plank in the top layer and near the right edge of the plank in the bottom layer (or vice versa)?
Not so much an invention, but knowing how to extract Vitamin C from sources such as rose hips and keep it shelf-stable would have made a HUGE difference once sailors began going on extended voyages.
Actually, I’ve been told that there was an easy way to get out doornails.
Burn the door and sift the ashes.
Seriously. In fact, sometimes they reportedly just burned the house.