there has been many stories of white foreigners being kidnapped in Myanmar. Could by why they are letting Thais in and not westerners. This is just what I heard I have no idea if it is true though
but you don't need a British marriage or British marriage licence. The Thai one works. You are legally married both here and in the UK. I'm not sure why you'd need a British license? I've never come across a need for one. Maybe I'm missing something though