I'm not sure how useful this is as I was a mature student and long-term British resident when I went to university. Anyhoo, I moved to a different LEA about 6 months before starting university in the same area. I applied for the full benefits from them and got it all so... I think that it may be more the case that she may have to have resided in the UK for 3 years rather than one LEA so, if you send her to school in England or Wales before that you should be okay.
As an aside, doesn't Thailand ask people to choose whether they are Thai or British when they turn 18, or is that just some BS that someone fed me in a pub?