I forgot, there is the London GC at Dunstable, which I understand also has a ridge, but I've never flown there, while I have flown from all the others I mentioned. Dunstable has an airspace problem called Luton Airport which complicates cross country flying.
In terms of cost, check for yourself but I suspect you will find that prices become less as you increase the distance from London.
Personally I would try the Long Mynd or Sutton Bank, both very nice sites.