As a progressive music fan since 1966, I share the idea common with other like-thinking folks have been around so long here in the UK, that jazz rock has always been part of prog rock. Having just seen Michael Figgis's pretty good movie "Red White & Blues" about the British blues boom in the 60's, I reminded of the importance of this blues movement and the way it evolved here, how influential those developments were in the early days of prog. As a reminder, the 1969 Decca sampler "Wowie Zowie: The World of Progressive Music" had jazz (Jon Almond) and blues (Savoy Brown's "Train To Nowhere") as well as the obvious Moody Blues track, something by Touch ("Down At Circe's Place") and Genesis (the most proggie track from "Revelation to Genesis"). There were no clear cut distinctions and none of the tracks there were considered out of place.
Again with the evolution of Soft Machine's music from 1966 to 1971 (listen to the first three legitimate albums and "BBC 1967 to 1971"), you can hear the move from Anglo R'n'B to psychedelia, to rock jazz, to jazz rock, to free jazz-rock. Least we forget there was an intermediate stage called rock jazz (which pulled BST, Chicago, under the progressive music banner).
And then the Nice - influenced by jazz as much as the classic. "Rondo" is Mozart out of Dave Brubeck. Some of the Nice's earlier long tunes had jazz musicians sessioning - the great Cornish saxophonist John Surman included.
Further I would strongly argue that a couple of seminal albums influenced the earlier UK proggers, Dave Brubeck Quartet's "Time Out" and "Live At Carnegie Hall -Vol 2". I went to school with a guy who soon became the leader and drummer of a minor UK prog band "The End" who were managed by the Rolling Stones. I know from asking him at the time, that he as many other aspiring drummers practiced to "Castillian Drums" (on the "Carnegie" set) with all of its varied time signatures.
I have no doubts of the connection. The problem is deciding: where does jazz rock stop and musically become some other form of jazz fusion?