In the past when UK ex-pats required an income verification letter from the British Embassy, you sent your income details along with photo copy of passport and a self addressed envelope to the embassy.This remains unchanged.
However, the method of payment has. You can no longer pay by money order and now have to pay by debit or credit card. I understand some people have been unable to locate the appropiate forms for doing so.
Google ” how to obtain a pension/income letter for Thai Immigration”. Scroll down to the gov.uk link and click on it. This will bring up the relevant forms. (It did a few moments ago !!) I hope this has been of assistance.