If you're in the Milton-Keynes area (about halfway between London and Leicester), you might enjoy this particular site --
http://mksessions.blogspot.com/.

It's geared towards performing (but not professional) musicians looking for places that offer live-mike, acoustic open sessions. Mainly folk and Celtic music.

In addition to talking about performing opportunities, the site also offers standard notation sheet music and midi files for loads of folk and session tunes. Well worth a look-see!