What Don says is right. But why should it only be expected of a professional musician? Why shouldn't an amateur who also intends to stand up in front of an audience (and a UK folk club audience is usually a paying one) also be expected to put in the effort?

Of course there are some who appear to believe that the purpose of a folk club is to allow people to do their rehearsing in public. There does seem to have been an increase in this type of club, but is it coincidence that folk clubs as a whole have declined over the same period?