The Cambridge CELTA and Trinity Certificate TESOL are the two most widely accepted qualifications. The next stage above them is the Cambridge DELTA and Trinity Diploma TESOL, but both require the first qualification plus two years experience teaching EFL.
A friend of mine did the CELTA course, and he said there were a few people with PGCEs on the course, and they didn't find it to be below them. I suppose it would depend on what you've been teaching so far too. If you've been teaching English, then you might be able to negotiate your way onto a more advanced course, but I don't know how flexible they are.
Another option for you could be to attempt to get a position at an international school instead. There are plenty of British, American and Australian schools which run off their own curriculums, where your existing qualifications might count for more. But I don't know how much actual EFL experience that would give you.
Anyway, here's the Trinity website, and the Cambridge website so you can read a bit more. Also, for a reason nobody's told me yet, the fact that the A in CELTA means "adults" doesn't seem to make a slight bit of difference to the age group if actually qualifies you to teach.
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