I believe that depends on circumstances. Like most things though, nothing is ever entirely clear.
It is my understanding, and clearly Nomad's too, that if an expat goes to the UK for the purpose of receiving healthcare treatment, then he has to pay. However, if his return is also to relocate to the UK "permanently", and receive healthcare, then it is available under the NHS free. Nothing of course, once having been treated, to change your mind about returning to the UK permanently, and coming back to Thailand. That would likely only work once!
Some expats never cut ties with their GP's, and some also make periodic visits to the UK, and make it appear to the authorities that they are still resident in the UK. Under such circumstances they have no problem receiving NHS treatment (as long as they join the queue behind all the illegals and refugees)
Those that rarely -if ever return to the UK, (and are known to be resident in Thailand (by HMRC, DWP and many others) will likely as not be refused free NHS treatment.