I really like the trail at Cannock better than Swinley/Chilterns etc Better than many of the better known routes in the north downs.
Although if you have an expert the North downs probably has more routes, but cannock you can just get on your bike and ride.
Dunno if its a “proper” ride but it is a good trail centre ride. There are rocks there (but not naturally) and a couple of nice black extensions to the trail which can be changeling.
Problem is its going to be over 2 hours driving probably.
Going to dorking is an obvious suggestion but if you train it there is still a bit of road cycling to do to the trails.
I think if you want to just go, get on a trail straight from the car park not think and have some good riding then cannock is probably the best idea. Also it holds up well in most weather.
Its a shame there are no forestry commission sites which are in between london and birmingham/bristol as they would be easily accessible for at all of riders. It doesnt need much altitude to make a good MTB trail just a few trees and plenty of weather proofing. MTB swindon are doing some good work on a small trail there…
Other places –
Thetford is quite quick to get to for you. But the trails are nt much cop in my mind (and not just because its flat)
Swinley trails are being redeveloped but there are a couple of good ones but again will you find them ? Might be better to go next summer when its all marked up if you dont know your way.