I fly with Redhill aviation, and completed an IMC rating there last year. Their standard of instruction is very good and they have a number of well maintained aircraft. Redhill is a great place to fly from, having ATC but still a relaxed feel about the place. You also have the advantage of the 'Redair' group, whom Redhill aviation belong to, also have bases at Shoreham and Blackbushe.
The disadvantages of the club are: Redhill has grass runways and can be closed for short periods during heavy rain (but then Shoreham should be open and it has hard runways). Also, on occasion the organisational side of Redhill Aviation could be stronger: I often turn up for a flight on time, and end up leaving hours late because the aircraft is at the other end of the airfield (parked by the tower for security) and it needs fuel. Basically, if you turn up far earlier than you think you need to, this shouldn't be a problem. If you need any more info, send me a PM.