Many years back I did by accident. I went to the British Embassy BKK to find out the procedure, outside was a tout, she said I can do everything for you including the marriage, that is all the paperwork, copies etc, start today, marriage certs tomorrow afternoon.
Cost 8000bht all in, at the time it was 70bht to the pound, so at a bit over 100 quid I thought a bargain.
All done without a hitch....????