Teaching English to people who can't speak any English or who are very low level, requires a different skill set to "mainstream" English teaching. Unfortunately, in Thailand a decent TEFL/TESOL qualification is not considered important. Teaching English to students who are at an International school or institute is more like teaching English to kids in the UK. Teachers at International schools/institutes are usually required to be qualified to teach in their home country.
There are hundreds of TEFL jobs in the UK as there's a requirement to teach immigrants (especially as the English language requirements are now higher) and lots of money to be made in teaching foreign students in the UK. In the UK a proper, specialist TEFL/TESOL qualification is required for these jobs, but unfortunately they're not particularly well paid.