Hacker,
If you are in North London try one of the clubs at Elstree, or even better try Skysport. Skysport is sort of based at both Elstree and North Weald, it uses Beagle Pups (the best light aircraft known to man) and Roger Hayes who runs the outfit is a jolly decent chap. When I lived up there I used them a lot for rentals and though I've not had a lesson there, I get the impression that the guys learning with them wouldn't swap.
They advertise in the back of both Flyer and Pilot, if you want, drop me an E-mail and I will give you more details.
T