The CAA does a very comprehensive list of schools and what they offer - available via their website or they will post you a copy.
Also try and get hold of the Learn to Fly Guide that one of the flying mags publishes each year (I have a copy somewhere.....) - it has a lowdown on what aircraft the schools have, which gives an idea of their size....ie if they can support a large fleet they are probably good enough to get sufficient students to part with their money.
Otherwise it depends on where you are based and what your requirements/ultimate aims are. Personal recommendations are always useful, and try visiting some of the contenders. If you're wanting to get into airlines, it might make sense to look for a
school that has good contacts with the airline industry. Not only big schools but smaller ones with instructors who are flying with an airline are worth looking for.
A.