So, from now on, I'll talk about the math-science qualifications for those who go to an International School or enrolled in an International program.
1. For IGCSE, you'll have to take Biology, Chemistry, Mathematics OR Physics. Your grade for these classes should not be lower than A for Chemistry and B for the rest.
2. For IB (International Baccaleaureate), you'll have to take Biology, Chemistry, Mathematics OR Physics, in Standard Level. Honestly, I am an IB full diploma student, and the document examiners will not be able to differentiate between Higher level and Standard level, so if you have a predicted grade lower than a 5 for your Chemistry 'HL' class, they will cut you out. So, keep up your Chemistry predicted grade at 6 and you will be fine.
3. If you are not from a Thai school or not of the 2 programs above, I bet Thammasat has yet to recognize A-Level and other programs, you'll have to go with the SAT subjects. You have to do three subject tests altogether: Biology, Chemistry, and Math II OR Physics. For Biology, you must do the molecular one (in case you want to enroll in other international programs in Thailand, I think they prefer molecular biology for the SAT). And your score must not be lower than 525 for all.
*But you don't have to go with IGCSE or IB if you are not satisfied with your score, my friend is in the IB program with me but she doesn't like her grades so she gave them her SAT subject scores instead, works both ways, choose one. I reckon you do SAT subject tests just in case.