Your first course should be to try and get in to Brize, this may or may not be possible. Well maintained aircraft good instruction off a variety of professional pilots who instruct in their spare time and excellent facilities. Did my first solo there.
Kemble is a good field to start from otherwise. Although I believe there is a question mark about it's future, a local resident has just achieved some kind of injunction which may limit the use of the field.
I used to instruct there four years ago. Two schools BCT and ASK. I worked for BCT, a friendly club, engineers on site and a decent field to operate from. Kemble was occasionally a little frustrating at weekends due to the number of fly-ins and aero days they had back then - not sure if this has changed.
Filton (Bristol) has two schools - this British Aerospace school is very reasonable priced but you will have to find a way in as it is primarily for employees. They have (had) a couple of PA28s.
Otherwise you have Aeros (who I also instructed for) they operate at Filton and at Gloucester Staverton. They are a professional flight training school who instruct to a high standard.
There were a couple of schools at Bristol International (Lulsgate) don't really know to much about those.
Visit a few see which you like, but I would recommed trying Brize or BAe first as they are or (were) most reasonable!