Stumbled upon this site whilst I was looking for gravel routes. It’s a US site but I don’t see any reason not to start adding gravel sections in the UK.
With the increased popularity of cross and gravel bikes, it could be a good route planning resource.

Gravel Map

Rightly or wrongly it relies on google maps data for finding paths, which means there’s some footpaths you can select and some it doesn’t know about.

Go to any area of FC land and that’s all you will find. An up to date OS map show forestry roads.

Go to any area of FC land and that’s all you will find. An up to date OS map show forestry roads.

The FC issue the data themselves:

http://singletrackworld.com/forum/topic/who-knew-such-a-thing-existed-forestry-commission-vector-files