Yes, much as I detest labelling music, I think prog folk fits with the Strawbs.
Others I think fit the genre off the top of my head: Maneige, Harmonium, Ptarmigan, Pentangle, Fairport Convention as mentioned, Renaissance, John Martyn, Clannad, Nick Drake, Lindisfarne, Steeleye Span..
http://en.wikipedia.org/wiki/Progressive_folk
Wikipedia doesn't do that bad a job of defining the undefinable. Anyway, love this genre of music, whatever it is.