I'm also from the UK and got married 18 months ago. He needs an affirmation of marriage and a certified copy of his passport. The affirmation can be completed on the embassy website and printed off but NOT signed, he must book an appointment online to visit the embassy and sign the affirmation in front of them and get his passport certified. There's not so many appointments available and when I booked it was a 3 week wait. I suggest going on the embassy website before he comes to Thailand to book the appointment first, then fill in the affirmation application online and print it off.
I went to phuket Town immigration for the list of required documents, showed my visa and ask about extention based on married to a thai , after consulting a college he said I can extend the OA when I asked again he said no can extend OA. I guess that means phuket is one of those offices that won't allow a change first time. Thanks for the information everyone.