Almost everyone commenting here (myself included) with have experience with only one school (the one they went with), and most seem to have a positive experience, so most posts are going to be along the lines of 'I went with School X, they were good because...'
Personally, I went with CATS, it took 9 months part-time (finished last month), and I achieved 14 first time passes with a 94% average. The instructors were knowledgeable and relaxed, the price is spot on, and the flexibility to chop and change subjects really suited my approach to the exams.
If I had spent £1000 more on 'School X' I would have taken about 9 months to complete and I would have (I hope) passed my exams first time with something around a 94% average (+/- 2%). I suspect I would have been happy with the result and considered the £1000 extra to be money well spent.
All of the schools know their stuff, the key factors in passing your exams will be to do with you (aptitude, motivation etc...), not the school.
Good luck,