So here are the IPAs listed in the 1977 Good Beer Guide, with their original gravities:
Eldridge Pope (1041)
Greene King (1035)
Whitbread (Marlow) (1038.2)
The average gravity of those beers was 1037.9. There seems to be a division between those brewers who used IPA to describe an “ordinary” bitter, and those who used it for a “special” bitter.
Charrington IPA was a notably sweet, lightly-hopped bitter that was almost the opposite of the original IPAs. According to the current Good Beer Guide it no longer exists, although the similar Brew XI does. And is Darleys now the least-remembered brewery in the country that survived into the CAMRA era?