I went to Bristol and will tell you they are first class. Instructors are amazing and the atmosphere down there is great. Professional instructing with some humour chucked in to get through those dull subjects.
I believe the guy who wrote the Bristol books also wrote the ProPilot books for the iPad, or something like that. So either way you're getting the same good quality from the books.
The 3 phase is good and I was looking for a 3 phase school too, but I managed to get a great average and first time passes from my first module with BGS, still yet to finish mod 2.
Heard good things from ProPilot too, they're exam results are brilliant. However, I think the school is relatively new and whilst the instructors are experienced I personally would go for the more established school.