What are the options for renewing a retirement visa in Thailand without having 800,000B in a Thai bank account for three months?

Does anyone know if it’s the same process for obtaining one from the UK Embassy in Bangkok for the financial statement please?

Just make an appointment $50

FWIW: I have used the US consulate's notarized affidavit of income from abroad letter for the last 10 years on my extensions based on retirement here in Bangkok and I have never been asked to prove the amount on the letter was backed up by anything (although I do have proof with me I don't give it to them).

I have seen people asked for proof when using those letters though at the immigration office but I can say it is very rare.

Notary services cost 50USD (or you can pay in baht)

You make an appointment online with the American Citizens Services branch of the US consulate in bangkok here;

In Bangkok you use this form for the affidavit:

/05/income-verification-affidavit-bkk-0517.pdf

There are three ways to meet the financial requirements to get a new yearly extension of stay based on being over 50 (which is what you're on).

Bank the 800K baht in a thai bank account for the requisite time period before your application

Get a notarized affidavit of income from abroad letter from your consulate stating you get 65K baht per month in income

Use a combination of banked money (seasoned the appropriate time) and the affidavit of income letter from your consulate to total 800K baht in a year.

Many consulates do not ask for proof when they notarize the affidavit of income letters and the US consulate is indeed one of them.

I will caution you that the immigration officer CAN indeed ask for proof that the amount you put on the letter is backed up by something (pension statement, bank deposits, etc). The chance of them asking is small but I wouldn't put down anything above the amount you can prove to them.