I recently used Bristol and I highly recommend them.
The school is run by Alex Whittingham who is probably the best groundschool instuctor that I have come across (I've met and worked with lots). The guy that teaches Met, Nav and Instruments takes a bit og getting used to, but their success rate is superb.
I passed all 9 of the national Nav Group exams after attending the 4 week course and a number of others that I have kept in contact with did the same.
ATA Coventry have a guy called Pete Lynes who works for them, who is, apparently, a bit of a star. Mike Burton also works for them and I can assure you from experience, his Tech subject lessons are superb.
I would say that its between Bristol and Coventry, I would'nt even consider the others. Having said that, its you're choice and I hope you make a good one.
Good luck whoever you choose.