Instead of the initial 800k can you just pay in 65k monthly. After 12 months apply for the retirement visa.
The advantage is As you pay in monthly you can spend or transfer the money back to high interest accounts. No need to keep a constant 800k in the account as long as 65k is paid in monthly.
Correct me if I'm wrong, the monthly payment option must be from a certified pension fund such as HMRC (UK state pension) or a UK certified pension scheme, the payment cannot be made from some individual's bank account. The problem is that if the state pension pays monthly into a non UK bank account this pension looses it's triple lock status, which means in the UK it will not increase with inflation (there are some countries that are exempt, but not Thailand)