I think Pete summarized the situation nicely. I would add that centres joining some kind of organisation is usually a good sign that they at least want to improve quality and/ or be known as a quality provider, as long as that organisation is not actually just another part of the same company- which happens!
A couple of other minor points. The CELTA is certified by Cambridge ESOL rather than the British Council, but is sometimes offered in British Council centres. While the standard of training on CELTA courses depends as much on individual trainers as it does on the organisation giving the course, having done your CELTA with a well-known organisation like IH or the British Council can't hurt as far as getting your CV noticed goes.
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