Without going into the 'ancient history' of English folk instruments, I would state that the traditional instument for English folk song is none. With very few exceptions, as far as I am aware,traditional singers sang unaccompanied. So England's National Musical-Instrument, as far as folk song goes is the voice! If 'folk song' is to accompanied does it matter then with what instrument? To my mind it does not;electric guitar, banjo,cello,concertina, church organ,Black Sea fiddle, aye and recorder are all as valid,or invalid depending on your personal view.My own view is, its not the instrument, but how it is used that matters.