Leadfingers' categorization fits what I see around here (Edinburgh/Midlothian).

There are also "open mike" events, somewhere between a singaround and a folk club open evening, but which are usually a different genre. If you want to do Mumford & Sons material or your own songs in that style, they're the place for you.

I don't know any regular event in Britain where unaccompanied fiddle is the usual fare, though. Does everybody in Jim's neck of the woods dislike Kevin Burke so much they won't play with him?