Will I face problems at UK check-in with a one-way ticket if I have a Non-O visa for Thailand?

No.

It’s a 90 day Non O based on marriage, then a 12 month extension for the same reason 400k.

Sorry to change the subject and ask a question, as I cannot find the site when putting my question,; just saw a video where a guy is saying that Bangkok Bank have changed the rules set out by immigration for money requirements for Non O visa, marriage and retirement and now the time required of said money , 800,,000 is longer than now hot does a bank change rules ste by immigration if the video is correct. Once again sorry to interfere in this person's question.

If you get any trouble at check in with your Non-O just ask to speak to the flight supervisor

IF you just received your 90 day single entry Non-O visa you won't be applying for ANY "marriage visa" once you get here 🙁

you'll get stamped in for 90 days on the current Non-O and then when you have 30 days or less you'll go apply for a year "extension of stay" based on marriage <- which is not a visa 😉

Be that as it may

you are unlikely to have issues flying in on a valid visa with a one way ticket. You won't have any problem with passport control when you stamp in showing the PDF print out along with your passport

BUT

You should check with your air carrier because they are the people that enforce onward travel rules. I would imagine because you have a valid 90 day single entry visa you would be okay comin' in one way, but check with them for sure