It is always dodgy to talk about 'exclusive'.
The traditions you mentioned (not sure about Welsh) were, in general, unaccompanied when we came to them, but there are exceptions.
The McPeake's accompanied some songs, though there is no evidence to suggest that was a part of their local tradition. Jane Turriff and (I think) Elizabeth Stewart accompany - judge for yourself whether an instrument adds to or detracts from their songs.
I always wonder whether accompaniment is part of these singers traditions or a personal idiosyncrasy. I think the fact that our tradition is basically a narrative one suggests that it was unaccompanied, but like much else of folk song, we simply don't know. For me, the benefits or otherwise of adding an instrument are to be found in the series of albums issued by Folktrax which have dubbed-on accompaniments. One of the conditions put to Topic when they planned to issue the Harry Cox recordings was that the practice be repeated.
Sensitive and skillful instrumentation can work, but quite often accompaniment does more harm than good to a song.
Jim Carroll