I've used the Pilot Centre at Denham for aircraft hire and have no complaints. The aircraft seem to be well maintained and well equipped (GPS and DME in most of them), and their prices seem to be competitive for the area. They also offer discounts for reasonable amounts of prepayment which could help.
The instructors seem friendly and knowledgeable, but I did my training elsewhere so I can't really say much about that; perhaps someone else could advise?
In any case VISIT SEVERAL SCHOOLS BEFORE YOU DECIDE, and don't base a decision on the nice pictures on a web site. Most airfields look pretty from the air, and after a few hours of circuits you'll get fed up with the view anyway.