It's a question. Those who are adamant that nation states ('England') have an indigenous and exclusive music that defines its 'folk roots' should see their observation is a nationalistic one. I think it's complete rubbish BTW. I don't believe there is an 'English' music, certainly not a folk one.
I'd like to know how folk 'purity' differs from ideas of ethnicity in this regard. There are differences but the two are uncomfortably close.
As for folk police - don't make folk laws and the accusation dies.