It's certainly true that the ethnic backgrounds of most of the people at the Folk Clubs that I attend tend to be (or have tended to be in the past), white English/Irish/Scots/Welsh with the occasional American guest. I suppose that that is mainly because these clubs usually celebrate white English/Irish etc. culture and not because people of other ethnic backgrounds are not welcome. Nevertheless, I can recall Indian, Russian, Turkish, Finnish, Latvian and Greek singers being received with great enthusiasm. I for one love to hear musics from other cultures.
Perhaps people from Britain's various ethnic minorities don't attend Folk Clubs either because they don't know about them or, more likely, they, - like vast swathes of the British population, assume that British Folk Music is not 'cool'.
Most of the folkies that I know are committed lefties and any hint of racism wouldn't be tolerated!