We have done ( I own a MT gym in chiang mai) about 20 from London. It's definitely not an embassy that asks too many questions. It's possible they may not ask for gym certification but can't guarantee it.
Aside from all the people who are using MT to get a visa don't forget there is a huge community of active muay thai students and fighters who the government consider quality tourists as they are studying a thai culture.
In the past some of those dedicated MT students would be chased out of Thailand for staying too long as a tourist. I think the government is trying to address this issue and as a muay thai gym owner we welcome it.
Cambodia just gave my client a real run around. ( I run a muay thai gym) they are probably going to start asking for a signed letter from the King soon ... I don't recommend them anymore