Hi Ben,
I can recommend EFG at Biggin Hill - finished my PPL there last year. Very friendly and helpful bunch of instructors, and seems to be a reasonably stable business. Also, having 3 or 4 planes of each type means fewer cancelled lessons (somehow, I didn't have a single lesson cancelled for a tech fault!).
Your best bet is to do a lesson at each of 2 or 3 schools (they will all count towards your final hours, and in any case you will be lucky to finish in the minimum time) and see which one works best for you. You'll save more by having an instructor/school that you get on with and that is flexible - this can easily make a difference of 10 hours to your total time.
Good luck!
J