I've done a CELTA course last year and actually I think it doesn't help with learning more about the rules of English or grammar (some aspects are mentioned, but the whole course is not a comprehensive review of teaching grammar...) That you have to do on your own, I'm afraid. Teaching wise: it gives you a framework in structuring lessons, where speaking is central. It's only a month if you do it full time with 4 assignments and I think 8 lessons (sorry, can't remember exactly). The whole course is well structured but it's really intensive! You can't really do anything else in that 1 month... Although I'm not sure it would help you how to teach higher level English (IELTS, etc.) I don't know whether the CELTA courses are exactly the same world-wdse, but I only had the chance to teach a Pre-Int and an Upper-Int class, 4-4- lessons each, which is not a lot + it's not exam prep you teach, just grammar, vocab, writing, listening, reading lessons with loads of speaking in each type.
I'm doing an applied linguistics course, which actually gives more insight into grammar/morphology, etc., nevertheless it's a year part-time and master level, so it's part of a degree, not a stand-alone qualification. If you're really into higher level En. teaching, maybe think about DELTA. With that you could teach non-native college/uni students, but that's super though, as I heard...
I'd be interested to see what others suggest. :) Good luck!
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