From what i understand from looking into this at the end of last year, you need around £
*****
in a UK bank account, an ACRO police record check & medical certificate. You will need to provide all this evidence at the Thai Embassy in London. You will apply for the visa one day & collect the following day.
The staff at the Embassy normally check to make sure you have all the correct documents before they allow you to process your application and take payment.
If this is the visa you want, do it before you leave UK. It's a lot more difficult getting the visa after you've moved to Thailand.