Good dirt climb or descent through the middle of Wimbledon Common. Can get a bit boggy in the wet so the wider your tyres the better, for both directions. A lot of fun on a mountain bike!
Epping Forest has a huge network of shared-use trails that are well-surfaced and wide enough to be enjoyed by pedestrians and cyclists at the same time.
Narrow chalk track with descent - deep gulley in centre with some big chunks makes first section interesting - suggest you need plenty of rubber to negotiate at speed. Wider and better surface after descent
Flat pack, chalky gravel. Can get a quite rough (cobbles!) at the southern end but can be ridden fast on a gravel or MTB setup. Mind the golf balls and the golf carts!