In judging whether the club is being reasonable or sensible, a lot depends on whether the rules are about the club as a whole or about the performer personally.

If, as you're implying, Cap'n B, the organiser is asking each person to contribute only songs from what s/he (the organiser) considers to be that person's background, that's a bit difficult to defend. I thought that approach had been laid to rest a long time ago, and I don't think a club organiser has the right to decide what's appropriate for the performer from a personal point of view.

Looking at it from the club's point of view:
IF it's a club in England - and you haven't said where it is - that strengthens their case. It's not unreasonable for an English club to concentrate on English material, or to have certain nights when they do this. It's not to be on 23 April, it it?

IF, on the other hand, the club is elsewhere (say Ireland or Scotland) I can see that a club that is looking for a varied selection of acts might invite someone who lives in or comes from another country to contribute songs from that country - that would also be quite understandable. Some clubs jump at the chance to get authentic performances of material from other areas or countries, to make a change from the residents' repertoire.

A lot depends on how the policies/guidelines are applied to other performers. If you could tell us that, we'd have a better idea what's going on here. If you don't actually know that, you may be misunderstanding what's being asked of you.

Marje