In Shetland there was the famous Peerie Willie Johnson, and in an old EFDSS magazine from 1962 I have a photo of a guitar player with the Jim Garson Trio from Orkney using a "steel", which it describes as "the Orcadian style".

There's an article on Musical Traditions about the Norfolk melodeon player Percy Brown which mentions his son-in-law Fred Devo sometimes accompanying him on guitar.

There were probably other examples of traditional players. However the guitar doesn't really seem to have been associated much with British folk music until the revival, which in its beginnings was heavily influenced by American folk. The current styles of "British folk guitar" are derived from the revival.