Why on earth should any UK based folk performer with roots abroad be excluded? Tunesmith? A Polish folk singer, to use your example, is still a folk singer...

Have a listen to this: Roshi - Jaane Maryam

The Rif Mountain website says this about her: "Born in Wales to Iranian parents, Roshi Nasehi is a singer/songwriter who presents her own evocative original material alongside sometimes quite radical interpretations of the Iranian songs she was brought up listening to... When she interprets Iranian song it is in a personal style bringing a contemporary edge and an authentic understanding of context and language."