Box Hill in Surrey can be found here:

http://www.streetmap.co.uk/streetmap.dll?G2M?X=517150&Y=151975&A=Y&Z=3

and is extremely popular on Sundays, although you can find bikes there every day of the week.

Numbers can get up to 1000 bikes in the summer so get there early (I'd suggest 10am and wait for the impromptu stunt show at the roundabout and along the dual carriageway.

There is a café there called Rykas whose slogan is "Probably the best burgers in Surrey".

It's on the A24 so is also a great starting point for runs to the south coast.

If you get herded around by an eccentric old gent in a white coat, then don't have a go, as some of the locals are quite protective of him and he is only trying to help.