This is an interesting point, re improvisation. The Boat Band that I play with certainly plays 99% trad folk, English and some American. We all improvise freely, always have. Not to excess I like to think, not around jig tunes for example. but certainly with song and tune backing. Folk musicians I have known(from many cultures) tend to be impovisers to some extent. It is the classsical people who tend to stick to the hard line.
As to the English folk revival, I never know what to think. Is it folk? Is it folk -related? Who knows what to call it.