As most people have said, it all depends, when it comes to details, but there's a pretty general assumption that there's a balance between local floor singers and imported guests, and that things should be pretty informal.

I've only come across one club where there seems to be a pattern of staged concerts and an audience there to listen. (Hitchin, the only time I got across there. Great concert with Eric Bogle, but it didn't feel like a folk club.)

Where the balance is drawn depends basically on numbers - small clubs are more likely to have occasional guests with most nights being singarounds, and so forth. Bigger clubs are more likely to have guest most nights, with a couple of floor singers.

One thing that's pretty universal - the guests stick around to listen to the floor singers, and the assumlption is that there's no big division between performers, paid or othwerwise, and an audience. Guests always sell their CD'sd, and that is often the major money spinner for them. And there is always a raffle for club funds. That's a tradition.

Function rooms in plubs have always been the main places where clubs meet, but they are getting harder to find. There are clubs in places other than pubs. There are even some clubs where you can't get a drink, I believe, but not many. (Even the Bishop's Stortford one that meets in a church vestry has a bar.)

Theoretically you have to be a member or a guest of a member to attend, but I've never heard of anyone being stopped from attending for those kind of reasons.

Any American visitors (for example) wanting a spot, just make yourself known to the person you pay admittance to, and you're pretty sure to get asked up. (Unless you leave it too late and there's no room for an extra one.)