Really This category is not suitable. Rock music is British - because there is a rock "community" covering, in my opinion, the British Isles. Even Dublin rock groups dream of playing in London to 'make it'. Folk music however is quite different in the different countries of the British isles. So there should be four categories (at least) : English folk singers, Scottish, Irish, Welsh, and no British category.
Naturally for rock , rap etc, this does not apply; it is the claim of folk music to be rooted in geographical idenities that make the separation necessary...