Don't quite know. However I do know of a secondary history teacher who teaches at an international school in BKK. Why not check what subjects international schools teach in secondary?
I think if you are only doing a PGCE to get a job at a "good" international school in Thailand, you will probably be disappointed.
Ask yourself, how many "good" schools are there? How many jobs are there in your subject?
There might be around five "good" schools and each school has 2-3 teachers for your subject (more if you teach a compulsiary subject like maths and science and less if you teach something like RE or psychology).
Factor in that some jobs will be taken by people who are long termers in Thailand with families etc. and other reasons not to move onto another country or school.
This means there are not that many jobs and each job will attract a lot of applicants as Thailand is seen as a nice place for an international teaching position. If you want to get into international school teaching you mght need to go somewhere else that is not so popular or build up experience in the UK first.
As it stands (as Exeter told you) you still need to do some undergraduate modules in RE to even be considered, that means you have to do 6 courses if they want you to have 50% of a degree in Religious Studies.
You can do a masters without a bachelor degree in the subject, I was accepted onto an MA History without a BA in the same subject but decided instead to do my PGDE in Business Education which also leads to an Masters in Education . I still hope to go back and do my MA in history at a later date.
For your information, it took me more than a few years of effort to get onto a teacher training course and this also cost quite a lot. If I were you, I would choose your subject carefully and then think about how you can meet the requirements.
If you want to do history you can do an MA via distance learning at somewhere like Exeter or Sheffield Hallam or do six modules from the University of London BA History and that will fulfill the academic requirements. This will take you a couple of years to do.
I would also try to get some experience in a (UK comprehensive) school to see if you like it before you even think about applying as this si where you would do any teaching placements.
Odi et amo. Quare id faciam, fortasse requiris? Nescio, sed fieri sentio et excrucior.