@David *******
if you are over 50 years old and happen to have more than the equivalent in Pound Sterling of 800.000.- THB in your bank account, just apply for the 90-days Non-Imm-O retirement visa. You will get stamped in for 90 days stay permit upon entry. Just use it like you would use a tourist visa, and fly back to UK on day 81. No extension needed, no border bounce needed, no onward travel proof needed