My experience in that neck of the woods is somewhat limited, or I'd have offered it. Redhill, the few times I've flown from there has always seemed fairly anti-aviation, but that was not recently and it may have improved.
Biggin I don't personally know, but it has a reputation for being expensive and a bit complex. How valid that reputation is, I don't know.
Both are in pretty busy airspace.
You could try Rochester (
Skytrek Flying School Rochester - Home page) or if you fancy Microlights Stoke (
http://www.ravenmad.co.uk/learn-to-fly.htm); neither I know much either, but they're in quieter airspace so may well be cheaper being further from London.
G