What documents are required for a Non-O Thai Marriage Visa when married abroad?

I think you got more than a few of your wires crossed..

Now while this isn't actually a visa question it's a how do I certify my marriage to a thai abroad one. Let's address it if we can

You're not going to get an in country Non-O visa issued based on marriage OR a yearly extension based on marriage if you married a thai abroad without getting the marriage certificate certified (in your case by the people in Milton Keynes) and then at the thai embassy (again in your case in London) <- You HAVE to have done that before you can even begin to get the marriage "registered" here thru the local Amphur (city hall) and get the Kor Ror 22 <- thai married abroad certificate because that's what you need to get both the Non-O visa in country and the year extension after that

The brit embassy here stopped certifying marriage from abroad a good while ago, so your only choice is to send the certificate back to Milton Keynes, get them to certify it then to the Thai Embassy in London, get them to stamp it, THEN send it back to you here so you can get it translated to thai, get it certified by the Ministry of Foreign Affairs, then go to an Amphur with your thai wife and register that you two were married abroad. Again the Kor Ror 22 is what you're after, but it's certainly a process to get.

If you wanna get a 90 day Non-O visa and year extension based on marriage to a thai and you were married abroad that's what you have to do so you can get the marriage registered here in the country and begin the process.

As I said, you're not asking a visa question, you're asking a 'how do I get xx document' to get a visa one and that's beyond the scope of the group. Post on a Brits in thailand group and ask, believe me you're far from the first foreigner who married their thai significant other back in Blighty who now needs to get that marriage registered in thailand so they can get a marriage visa/extension.

Unfortunately as it's beyond the scope of the group I'm not going to even let the thread open for commenting, but I at least wanted to get you pointed in the right direction. I also messaged you about

Best of luck with it