It certenlly depends on wich group. I don't think Fairport or Steeleye could be considered prog: They invented fol-rock, but never went prog. But some of the groups you mention could fit into the prog genre. The key is that prog shouldn't necessarilly mean 'weirdness'. It could be a more complex way of composing the drum part (folk drums are pretty straightfordward and it's difficult to apply complex rythms to it), or more complex song structures. Mellow Candle has very Jethro Tull-ish drums. Carol of Harvest (a german band, but in the English tradition) had long suites. Magna Carta were, for at least a couple of albums, prog, with the 'Seasons' suite and the the 'Lord of the Ages' epic. Loudest Whisper (a group a don't like) had some classical influenced tracks, as well as a two part record. There's one obscure group called 'Pererin' that featured a very moving Camel influenced guitar in a couple of clearlly prog instrumentals...
That is, it depends on which group we are talking about. Usually, most of these groups weren't pure prog, as many tracks remained primarilly folk. But those who dared beyond electrifying their instruments should be noted in any prog page or discussion.