Three jams in London advertised in the FOAOTMAD magazine The Old Time News:
Balham: 1st Sunday - The Grove, Oldridge Rd. Call Dave East (020) 8677 9989
Camden: Monday - Hemingford Arms. Call Caspar Cronk (020) 8340 4683
Islington: Sunday - Harlequin, Arlington Way. Call Frank Weston (020)8552 830
I've not been to any of these, so I don't know what they're like, but they're not necessarily pure Old Time or Bluegrass. I do know that Caspar Cronk plays great musical saw!
Cheers
Ray