I’m not excusing the OP from seeking legal action, I’m just painting the scenario he’s thinking might happen. People get defensive when legal documents are involved, even if it’s a good legal document like the deed to your dream house!
Even if the ex is a wonderful woman, she’s going to wonder why now he’s getting lawyered up.
Yeah, but nothing that the OP has written suggests that his ex would do any of that. Denying him visitiation just because he wants more legal protection? If she’s that irrational, that actually strengthens the case for him getting a lawyer involved, because it means she can’t be trusted to look after his and his child’s interests.
True, but again, a notary is a complete different kettle o’fish. The OP and his ex call a notary for an appointment, and state their joint wishes. Notary draws up a contract, slighly adapting it from a standard contract. Op and ex have a second visit, where they get explained the documents and have a couple weeks to think it over and mail adjustments. A third visit can be brief, where the (ex) couple both sign the documents and each gets a copy. There, done. In a month. Cost: between 500 and 900 bucks.
This is not how it works in the US. The only power a Notary has in the US is to witness signatures (usually it is someone like a Bank teller). it would be considered unlicensed practice of law (a crime) for a non-lawyer to draw up a contract and advise clients on its meaning (unless they were directly supervised by an attorney, who verified all the contents of the contract, and signed his name to the work).
I’m not sure about the UK. I know they have the Solicitor/Barrister distinction. I would doubt that have the Civil system conception of “Notary” as well.
But you are correct that nothing stops anyone from drawing up a private contract about custody.
This could be jurisdictional. For example, it may be that there are no “sole notaries”–all notaries are also lawyers. In which case, he might as well just go to a lawyer. This is certainly the case in my jurisdiction (Alberta, Canada); and given that our practices descend from the law of the UK (where the OP is), I wouldn’t be surprised if it was the same there, but I don’t know for sure. Regardless, there may be very good legal reasons why a notary is not an option for the OP.
All of this is absolutely true. I do think the OP is making mountains out of imaginary molehills.
But the bolded part was not what I was facetiously hinting at. Putting myself in the ex’s shoes, I wouldn’t sign any such paper. I’d reason that I like the guy now, we’re still friends, he’s a great guy, we still get along marvelously, everything is hunky dory. I have zero intention to ever try and jinx this.
But.
But maybe someday he’ll change, or start drinking, or become an asshole somehow. Maybe someday I’ll change. Or maybe I’ll find an even better father for the tykes, or one of us will move and maintaining the statu quo is going to be a PITA, or any number of unforeseen, unplanned stuff. Life is chaos. And since I can’t foresee any of this, I’d rather keep an ace in my sleeve, just in case. Even if I never plan on using it, why give it away ?
Yes, I might be a bit of a control freak 
That sounds like a pretty good situation for your son. 50/50 custody arrangements are not actual always ideal for the kids - can’t go to scouts with their schoolfriend who lives near their mum because they’ll be staying at their Dad’s that night, don’t have their English homework with them because they did it at Mum’s and left it Dad’s, have to either not have their favourite teddy-bear with them at bedtime or take it to school and get teased for it…
Living in two homes 50/50 can be really awkward. Stability is the key. And you’ve got that, so well done. 
I think you’re right not to go to court, and you’re definitely also right that going to court is not the usual thing in the UK. It’s not even recommended in divorces where there’s a child involved - they’re advised to go to arbitration instead.
Putting something down in writing now would be workable. It would also be very helpful if your ex were to die and there were any question about who has custody of your son. It doesn’t have to be written by a lawyer to be something that would help you in court if it ever came to that.
I know you’re worried that courts don’t enforce custody, but they usually actually do - it’s just that a few really bad parents ignore the courts and put up with the sanctions on them. Most go along with their rulings. Since your partner is reasonable even without a court forcing her to be, I don’t think you have anything to worry about.
One of your main fears can be settled easily: it really is not that easy to take a child out of the country without the other parent’s permission these days. The reason the law was strengthened was due to child trafficking (real or imagined) and actual cases of parents, mostly fathers, taking their children to another country and never bringing them back.
My daughter’s father has declined to ever have any contact with her, so there’s no way I could get a letter from him stating that it was OK to take her on holiday, which is what you’re supposed to have when taking a child out of the UK on holiday. I was still stopped at immigration in Canada and asked lots of penetrating questions for about twenty minutes. My daughter’s 13 and well able to speak for herself, which was one of the reasons we were finally let through. We also only had tourist visas, had our hotel booking forms, and the details of our return journey, all of which we had to show.
Perhaps what also helped was that I answered honestly when asked how I’d met the friends I was going to be staying with for part of the trip: ‘on an X-Files messageboard years ago.’ :o 
God, I was so sure that saying I’d met them on a messageboard would make it sound like I was about to hand her over to a child sex ring or something, but from the official’s demeanour I think answering that honestly made her realise that I wasn’t likely to have lied about anything else. Or maybe she was a fellow fan!
Next time I’m going to make sure I have a copy of her birth certificate, which has no father listed, and hope that will be enough. But they will still not be obliged to let us through - we could be sent home on the next plane.
Anyway… you only hear the horror stories. Stories like yours don’t make the news. I understand you being worried, but in your case you have very little to worry about.
(And I bet you are still reading).
We don’t have notaries. TBH, well-meaning as you all are, I don’t think specific legal advice from people not in the UK is very useful for the OP.
You are more than a bit of a control freak; that is completely fucked up. My jaw actually dropped. Maybe you’ll find an even better father?! Jesus, I hope you don’t have kids. And if you do, I hope your partner knows you have the soul of a monster.
I looked up UK separation, and there was this:
http://www.hartnellchanot.co.uk/unmarried-couples/unmarried-couples/children.html
It seems to be pretty similar to OZ where there are low cost processes to come to a formal agreement on child care and the like. It also looks to me like there are quite a few processes to ensure Biffer will be involved in their lives unless safety or the like is involved.
From what I read , Biffer is also following the correct process, and the time to go to mediation or the like is when things break down, or become difficult, not as a ‘proactive’ action.
Otara
I don’t think it has “earned” that reputation, but it certainly has been smeared with it.
According to this wiki article, there are various types of notaries in England & Wales.
Oh, it was earned. Historical revisionism aside it wasn’t so long ago that the hatred of men, of sex, and of anyone who disagreed with orthodoxy in any way was the public face of American feminism. I have no idea how dominant such people were in terms of numbers, but they definitely managed to dominate in terms of public image. If they were smeared, it was mostly something they did to themselves.
Then I suppose your shrill overreacting soul will be relieved to know that I don’t have, nor ever intend on spawning.
During that era “politics” meant dealing with the cold war, race relations were defined by segregation, and a healthy diet focused around eating a lot of red meat.
Stuff changes. Do we give doctors shit because fifty years ago the medical establishment gladly said smoking was healthy? Do we give George Bush shit because of the McCarthy trials? Why is feminism, and feminism along, confined to the past?
Say what? I was talking about the 80s, early 90s. They didn’t start the “sex-positive feminism” movement back then because feminism as a whole had a pro-sex attitude, after all. And it was after the Cold War that I recall the Russian feminist delegation storming out of an international feminist convention because of “American Stalinism”, which was nicely ironic.
I am a non custodial father in CA, my arrangement like the OP is completely informal, as it has been for 13 years. At the same time, I keep a journal and document everything, print out copies of checks, keep receipts for clothing and gifts I purchased for them, and track the time I spent with them. Only takes a couple minutes every week.
I do this because after breifly consulting with an attorney on the matter, was informed that such a thing is admissable as evidence in the event mom flips out and says “he never sees the kids, he never paid a dime towards their care, I want 13 years of back child support”.
So even if a court decides I should have been paying $400/month not $350, there will not be a huge pile of arrears because I can demonstrate that there was some kind of arrangement and or effort on my part to contribute which I have consistently complied with.
Despite the tone of your post even sven I think this is an ideal arrangement, courts and formal custody orders are for people who cannot play nice because they are more wrapped up in their own squabble with the ex to come to a reasonable agreement about the children. It can also be adjusted as circumstances dictate without months of lawyers and courtroom appearances. Legal solutions also tend to be kinda cookie cutter and can be highly disruptive to everyone because they dont mesh with anyones life. I have a friend who her custody orders specify hand off times and what holidays who goes where, etc, etc, its like a 25 page legal document spelling all this out, it never works well, and cops end up being involved every few months over it.
As opposed to me an my sons mom, hey Drach, we want to go camping next weekend, would it be ok if you take the boys the weekend after instead. I reply, sure, have fun! We give each other as much warning of any major events as possible and everything works out fine. If it comes to any kind of conflict (which has only been 3-4 times), I pretty much just bow to her wishes, she provides primary care, she gets first consideration. She has never in my view abused that consideration.
If she decided to leave the state or something we would just find a way to work that out as well.
I apologize. I think what you wrote is pretty screwy, but I don’t think you meant it that way, and if you don’t have kids I think maybe you don’t grok exactly why it’s kinda fucked up. Or maybe you have a different idea in your head, or something; whatever. I’m sorry. I cannot agree that it’s ok to say you might find a better dad for kids, but I’m pretty sure you don’t actually have the soul of a monster. So yeah, overreaction on my part.
It was a small minority consisting of extremists, kind of in the proportions of Al Qaeda types compared with everyday Muslims. They just got a lot of press because they were outrageous, and they fed into the fears of anti-feminists. Sort of like what happens to every group that wants to change things. Most of us thought those women were idiots.
Well that’s different, for the Pit. And yeah, a day later I see I probably worded that in a fucked up way. Or at least more fucked up than I actually am. So we cool.