Still some MTB riding to be had, so don’t discount it. I did same thing as you and live centrally, ride MTB and gravel, Zwift on my balcony, and use the train. Ideas as follows:-
1. Old Eastway circuit at Stratford has a competition standard XC MTB course. Keep heading north and you will get to Epping.
2. BMX tracks can be ridding on MTB’s
3. Get an OS map (1:25000) and start linking parks and other urban spaces up. I can ride from central London to M25 eastbound without riding on a road.
4. Train from Fenchurch St gets you to Benfleet where the 2012 Olympic MTB course is
5. Swinley from Waterloo early is good, never had issue with muddy bike on way back.
6. Train from Victoria to Warlingham gets you on decent single-track.
7. Google / look up on a map the Green Chain Walk, most is legally rideable and will give some options
8. Oxleas Wood / Eltham Common has some decent single-track
9. Cyclopark at Gravesend (train from Stratford / St Pancras is 20 min, and there is some good bridleway stuff nearby
Don’t get me wrong, it’s no MTB Mecca but with some thought, a change of tyres (as XC lightweight as you can), and determination there has been enough to keep me going over the last 6 or 7 years.
Good luck!!