Ruislip: What is it good for ......?

You'd get stuck between the trees, though.

You sure can.
The bridlepaths, indicated on the 'You are here' maps at (major) entrances, are for use by cyclists.
During Th'Shutdown many more people have been visiting th'Woods, and unfortunately (for the lesser lifeforms), many more (maintained) footpaths have been used by mtb'ers.
You are unlikely to be stopped, as 'enforcement' concentrates on parking infringements.
Enjoy yourself, be careful, and watch out for exposed tree routes.
We have yet to determine a policy or practises to reduce erosion.