You mean sessions in places where there are festivals, when there isn't a festival?
My impression at most good festivals is that somewhere in there you find a session which is largely made up of people who are based locally. But in some cases they aren't exactly local to the place where the festival is happening - so my understanding is, for example, that there's not much of a folk scene in Sidmouth, but there is in Exeter.
Whitby I believe has a lively enough folkscene, and so has Fleetwood (Fylde). Of the smaller festivals, I know that Walton has aa good regular session - last year, when the festival was cancelled, it expanded into a mini-festival in the Royal Marine, and got the momentum back for a full-scale Walton Folk Festival again this year.
A lot of clubs and sessions in places where there aren't festivals tend to die down during the summer, or even pack up completely, what with people going on holiday and festivals and so forth.So the winter is really the on-season for sessions.
So roam as you rhyme...