When a track is marked as 'off-road bikes recommended' on the map, it kind of piques my interest. For this reason - in an effort to scratch a gravelly itch - I headed out last week to the Southern end of National Cycle Route 21 at Biggin Hill, which takes in quite a few miles of rough bridleway/singletrack before hitting tarmac. (Edit: it's not really the end, it runs all the way to Eastbourne, but it looks like the end because my map stops.)
For maximum un-paved points I also took in Riddlesdown which, from the North, is a real treat, but do watch out for pinch-flats... ahem.
The full route on Strava. Managed fine on my road bike with 25mm Panaracer Paselas.