Would be interested to see a list - I've always been under the impression that, barring a tiny handful of traditional singers, in the British Isles the tradition was overwhelmingly unaccompanied.

I was thinking of Bob Roberts, John MacDonald, Davie Stewart & Jane Turriff. Do the McPeake's count? And Margaret Barry? I'm sure I've heard of others too here and there. I was hoping you'd maybe be able to flesh it out a bit. Elsewhere in the English folk song tradition (America / Australia) it's not so uncommon; and singing with instruments seems to have been a pretty standard aspect of human musical activity the world o'er these past 50,000 years or so, so I'm not doing anything too radical.