Folk clubs restricting themselves in any way is a fairly new invention. The Watersons' early incarnation at the Bluebell booked, among other people, Sonny Terry and Brownie McGee. It was only when they left that people starting talking about their "legacy", and booking only folkie acts. They were dismayed.
The point of the snake should not be that it eats its own tail. Some of the best folk clubs I've ever been to have had a lovely, varied bill of music from all cultures, poetry and performance. Folk clubs do not a tradition make. Proper folk and traditional music should be out and proud in the wider community, readily influenced and influential in turn. Be an expert in your own field by all means (please try it!!!!), but don't shut yourself away from the world. To freeze English music where you have it marked would be to base a tradition on only forty years of recording and a load of unsubstantiated guesswork, and to exclude the other people that have lived here alongside us for hundreds of years now. Surely that isn't right. Nobody knows what folk music sounded like two hundred years ago. I love that, it means we can make a new, informed sound now.
Personally, I have always had a problem with people singing in any other accent than their own, trying to mimic someone else's culture in the absence/ignorance of one of their own. But I recognise that the only way to get them to understand what they potentially have, fulfill the thing they may be missing in their lives, is simply to be as good as possible, win them round with kindness and excellence rather than proscription and lecturing.
David, have you ever investigated the music that Greg makes in his community? That is truly the way to do it. And if you want to cook old English food and offer it round as well, that's cool, it really is. But I'll have some Indian chutney on the side of mine, ta. Pork pie and Patak's garlic chutney go really well together, and neither ruins the other's taste, provenance or importance.