I think that the school that you have been in touch with are being a little pessimistic as far as an ED visa is concerned.
Yes, some of the offices have tightened up a little, but here we are talking about people that have been on the visa for years & yet cannot hold a conversation in Thai with the officer concerned.
I have not heard of any issues with a person trying to get the visa for a first year while attending a reputable school. The problem in the Udon area might be getting a school that satisfies the Thai authorities that it is a reputable establishment. That used to be AUA but I understand that for the moment they have given up on teaching Thai to foreigners & so are not a route that you can take.
I believe that the Universities in the area (Udon & Khon Kaen) would be able to help you out.
You could certainly get a triple entry tourist visa from the consulate in Hull. That would allow you to stay for just about 9 months in Thailand, with careful use of exit time & in Thailand tourist visa extensions.
I hear what bigsnake says about only being allowed to convert non Imms type 'o' to a marriage visa but I am not sure that is still actually the case & you would, sensibly, need to check with the Udon Thai Immigration office once you are here as to what exactly they require of you.
I say that having a non 'o' might be no longer a requirement just because if you are under 50, not wanting to conduct business in Thailand & not wanting to go into full time education then it is actually impossible to obtain a non 'o' from the UK now.
If a non 'o' is a requirement then I believe that you could obtain a 3 month one in Vientiane once you & your wife legally married in Thailand. That could certainly then be converted to the 1 year, report every 90 day variety in Udon Immigration, given that you satisfy all the hoops & jumps deemed necessary on the day.