I used to live in Guildford, and work up on the North Downs at Holmbury St Mary. There was some wild lunchtime MTBing, but I always felt the roads were too much of a racetrack to risk cycling. For US readers, the N Downs are 30 miles from London, in the "stockbroker belt" . The roads are too narrow for 2 regular UK cars to pass, so there are crossing points every few 100m. When a 4x4 and a small car meet, the small car gets off the road.
The smaller rural roads are old winding horse cart tracks, worn down by thousands of years of use, so sometimes they are edged by earth banks a few feet high. In places the banks are 10ft high.
They are very scenic and test your driving skill to the limit.