Everything is detailed on the website. If she has a job or definite reason to return to Thailand, getting a visitor visa is straightforward. If she has her own money and can prove she can pay for the whole trip it's easy. Loads of Thais visit the UK as tourists. If you are sponsoring her (i.e paying for the trip) you will need to show you can do this.
I've had friends on Ed visas for studying French, Japanese, any university degree, Muay Thai. All depends on whether the school/course meets the requirements.
It's quite efficient (takes about 2 weeks) although it helps if you do it while in the UK. The Legalisation Office will forward the legalised document to the Thai Embassy for you; you just need a pre-paid envelope addressed to the Thai Embassy in London SW7, and then another pre-paid envelope for the Thai Embassy to send the document back to you. The Legalisation Office charge 30 GBP and the Thai Embassy 10 GBP, although for the Embassy it can only be paid in postal orders!
I expect the UK marriage certificate will need to be legalised by the UK Legalisation office and then this has to be certified by the Thai Embassy in London. I had to do this to get my wife's Extension as my dependant. If they mentioned the Thai Embassy in London it will be for more than just certifying the translation. Interestingly, Cheangwattana didn't require our UK marriage certificate to be translated; just legalised and certified.
Noted. My passport doesn't expire, it's just nearly full. So I'll try and time getting a new one to coincide with doing my 90 day and Extension renewal at the same time in the new passport.
Great. So I could do my 90 day as normal in July, then get the new passport, then go to Immigration in mid-Oct and renew my Extension and do the next 90 day in the new passport.
Thanks. Can I wait until the Extension is due to be renewed and then just get the new one in the new passport (thus avoiding an additional trip out to Immigration for the transfer)?