Vote number 1 for the British Airways Flying Club at Wycombe.
Instructors aren't there for hour building purposes, they're career instructors with shed loads of experience.
Prices may not be the cheapest but wherever you go check for hidden costs, ie landing fees, fuel surcharges, ground briefings etc etc. BAFC are part of Airways Aero Associations Ltd who operate the airfield - the prices you get are what you pay. Check how the hours are charged for - chock to chock, airborne time, engine running time....
The aircraft (7 PA28-161 Warriors, 1 Chipmunk and 1 Piper PA28-236 Dakota) are amongst the best you'll find on any flying club school fleet and are maintained regardless of cost, safety is paramount. They are maintained by their own engineers on site who also look after 25 private aircraft. The big fleet of Warriors means you would be very unlikely to lose a lesson in the rare occassion there's a problem with the aircraft.
Visit all the places you are thinking of learning and even have a trial lesson at each. Make sure you have a log book and record the flights as trial lessons count towards your 45 hour minimum for the PPL(A). Make sure you ask to be shown around the facilities including the aircraft and the engineering set-up (we always welcome prospective customers and customers alike here).
The PA28 Dakota has a Garmin 430 GPS fitted but as it isn't part of the training syllabus, you'll rarely find training aircraft fitted with them, or private hire ones either for that matter. Most people have their own handhelds that they can take home for route planning etc.
PM me if you have any other questions.