to get the visa, it's correct. But that's only 90 days and from the Thai embassy in UK.
Once you're in Thailand immigration will only accept 800k in the bank or embassy verified income. The UK embassy stopped offering this many years ago. That means the only option is 800k in your Thai bank account.
it's been clarified about 500 times in this group. And the list of requirements immigration gives out says it as well. And it will probably be stamped inside your passport too.
You say non-O. If you're trying to convert from tourist to non-O visa inside of Thailand at an immigration office, the 800,000 must be a foreign transfer.
If you're talking about the annual extension (not a visa) then it does not need to show as an international transfer.
For ongoing retirement extensions, your account must be at 800,000 for 2 full months before you apply for your extension, and remain at 800,000 for 3 full months after you receive your extension.
The bank account can never drop below 400,000.
That means it must be at 800,000 for at least 5 months every year and can never drop below 400,000 for 7 months.
If the balance drops below these amounts even for 1 day, you will not qualify for your extension next year.
Most people just leave the 800,000 in the account all year so they don't have to try to remember when to bring it back up to 800,000 and risk losing their extension.
That's up to you. Just know your bank account could get locked at any moment forcing you to go into the bank with all your documents and potentially waiting weeks for it to be unlocked. And when you want an extension from immigration, if the money for that extension is not in your account for 4 months when you ask for the letter, they will require you to freeze the funds for an additional 4 months from the day you get the letter.