Your work permit is now void you cannot work on a DTV. Getting a bank account will be very difficult as you are now just a glorified tourist with no real need for a bank account inside Thailand.
For a Non-o based on over 50 there is no need for proof of relationship in the application. You have applied for the wrong category. You only need to show £18k as a final balance on your latest bank statement and a photo of your UK driving licence for proof of location