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.
Well done I'm still awaiting my approval ? Submitted on the 21st June, all docs checked and has been awaiting approval now for 5 days!!. Can't see a contact email to ask the thai embassy if there is an issue?
as John says you can extend in thailand but for this extension I.e. after the 2 years if you get your timing right you will also need a thai bank account with the 800k deposited aswell as the mandatory insurance