Depends how fit you are really. If you're pretty fit then taking the train down to Dorking (I hop on the train at Clapham Junction), riding out to the North Downs to join up witht the Nirvana trail ride shouldn't be a problem. Would end up being about 3 hours of riding though, so you need to be fit.

Depends where you live too — south east London isn't easy to get anywhere good, Southwest london — Swinley and North Downs are easy by train. North West London — Swinley. North East — Epping is riding distance.

Have a look at some clubs too. VCL based at Herne Hill concentrate on track riding, but they've got a really active youth/junior section and they do quite a lot of trips to places like South Wales. They'll help you have a go at tack racing too, with a borrowed bike and stuff. It's a really good laugh, loads of MTB'ers turn up at their Saturday morning sessions during the Spring/Summer.

Here's their link: http://www.vcl.org.uk/offroad/
In east London team economic energy are pretty active — do a search for them.

Lastly, keep an eye on www.londoncyclesport.com and have a look at www.beastway.com, which is an XC race series held over the summer on a wednesday evening.