When you are applying in your home country you use the equivalent of 65k baht a month in your own currency or 800k baht total equivalent again in your own currency. So if in the USA this is in dollars in your own account. You only need a thai bank account when you are wanting an extension inside of thailand.
I contacted the London embassy about this exact question as I travel 3 times a year for 2 months each to thailand so not there long enough to get the extension. The London office said that there is no limit as long as you meet the criteria
no body knows what they might do with DTV's in the future. The soft power DTV that seems to be the most favoured in the muay thai training typically you would need a course for 6 months or at least confirmation that you have signed up from an approved muay thai gym, others have been medical treatments ( more than one). These have ben successful in gaining a 5 year DTV
Best to apply for a non O over 50 retirement visa through the evisa system online before yiu travel. You cannot open a thai bank account from the uk you have to be in thailand and physically go into the banks to see if they will open an account for you. Your best chance is having the O visa before you arrive. You will also need to have a thai telephone number/sim card and get a residence certificate from immigration using your TM30 from your place of stay. If you want to get the yearly extension you will need to deposit 800k baht into your new account within the first week or 2 to give it the 2 months required to season. For the non O in the uk you can show the equivalent 800k baht in uk pounds from your uk account or 65k baht monthly income in uk pounds.