Yes you need 800k and where you are applying for the Non-O determines where that money should be or if you can use 65k income or Embassy affidavit if available. Applying in the UK the 800k can be anywhere in your name.
You get your retirement visa (over 50) before entering Thailan in the UK, open a bank account soon after arrival then deposit 800k to season for 2 months to apply for your 12 month extension (not a visa)