For what it is worth, the earliest use of the term ‘pub rock’ I have found is in the November 1973 issue of New Society, the British sociology magazine, in an article by Andrew Weiner titled “Rock to the Top.” He identifies Brinsley Schwarz (named after the band’s founder), as the first pub rock band, which started as a progressive band but, after a while, ‘They no longer wanted to "progress." They played much simpler music now, country-rock, and soul, and straightforward rock and roll. Finally, Brinsley Schwarz began playing in pubs, starting with the Tally Ho, and slowly establishing a circuit…’
Weiner also writes,
Clearly, the pub-rock boom was as much a consequence of a decision of a section of the rock audience as it was the result of the actions of a few musicians. Pub-rock began to grow just as the great "progressive" boom in English rock began to peter boom in English rock began to peter out in in a welter of eight-minute guitar solos and rain-sodden mass festivals. Like the new pop music of the teenage idols, such as David Cassidy, David Bowie, pub-rock was a kind of reaction to the fake profundities and cosmic inanities and pure dullness of most progressive rock.