I did all the sums when this first broke and if I put your figures into my fag packet calculator you will currently be paying ~ £4,000 GBP income tax per year. If all your remittances were to be taxed in Thailand, after various allowances you'd be paying the baht equivalent of ~ £3,000 GBP per year.
As the DTA allows you to claim back tax paid elsewhere then you owe the Thai taxman nothing (although you can't claim back the "overspend").
As I say, this is all fag packet calculations so if anyone sees anything blatantly wrong I apologise, but let me know what it is and I'll see if I can make more sense of it.