Do I need to transfer my funds to a Thai bank account for my OA Retirement Visa extension in Thailand?

The gross amount of a pension is irrelevant it's the net amount that would be important assuming you could get an income affidavit which you can't. Once you're OA visa has run it's course you have 2 options for an extension. Either have 800k baht into a Thai bank 2 months before applying, it has to stay there for 3 months after and can't drop below 400k at any point in the year. Or transfer a minimum of 65k baht each and every month for the full 12 months prior to applying for an extension into a Thai bank from overseas, it must show in your bank account as being from overseas but once it's in the account you can spend it.

To use your pension in your home country, you need to get an affidavit from your embassy in Thailand. Unfortunately, UK wouldn't issue those affidavits anymore. That's why you, as UK citizen, need to have a Thai bank account to transfer your funds to meet the requirements for 1-year extension based on being over 50.

BTW, IF your visa validity ("must be used by date" on your eVisa PDF) is still remaining, you can re-enter Thailand on that valid Non-OA before that date with valid insurance (1 more year) to get stamped in for a whole new 365 days. Check your Non-OA visa itself, NOT the date on your entry stamp.

Start transferring 65k each month into your Thai bank account. You can spend this money as soon as you get it. You'll have 12 months worth by the time you apply for the extension so no need to tie up 800k

Your 1-year OA can be stretched to 2 years if you re enter Thailand just before the 1 year expiry date is up, then you will be stamped in for another year from the date you arrive providing your required insurance cover also extends through the 2nd year.

After that to extend in Thailand, your funds (800k THB) will need to be in a Thai bank.

Everytime you enter Thailand you get stamped in for a year (or for the duration of your insurance if less) so just before your OA Visa expires (not your entry stamp) re-enter Thailand