how long is the letter valid for once you get it ...up to a month or not? (for the immigration use)

One month should not be a problem - even longer is often allowed.

Up to a month is no problem, from my experience. My friend got away with a three months old one but there were some Q's and raised eyebrows from the immigration people

If you apply to the UK consul in Chiang Mai she will only send your application to Bangkok. It's usually very difficult to even get her on the phone much easier to deal with the Embassy directly