Brick lane bikes may be worth a visit, if you're around Brick Lane. Seen a few 2nd hand track frames there during visits.
http://www.bricklanebikes.co.uk/

Would 2nd the opinion that you've got to be careful buying bikes at the market. Lots of hot bikes there and the last thing you want to do is ride a bike ripped off from a London Messenger or one of his mates.

Second-hand frames are hard to find in the UK IMHO. Its a pretty tight market with lots of buyers and not a lot of stock right. Just have to patient as the supply is pretty steady.

Keep an eye on the Moving Target Zine site. Every now and frames are posted.

Best places though seem to be the Bike jumble/swaps. Ripley is really good if you can get there (next ripley jumble is March 31st). The Good Friday swap/meet at Herne Hill should be good too (6th April)