The full list of who’s playing on this is available here - http://www.folkways.si.edu/classic-celtic-music-from-folkways/album/smithsonian.
Why the album’s compiler thinks Billy Pigg, Shirley Collins or Ewan MacColl were ‘Celtic’ is utterly beyond my ken.
Very true; in fact, they covered Northumbrian instrumental much pretty well in their CD "Northumbrian Rant". Maybe those folks are not Celt-IC, but they are Celt-OID. (a friend says that "Celtoids" sounds like a skin disease).
Finally, I have never gotten the appeal of Shirley Collin’s breathy voice, but she seems much beloved by the English folkies.