If good timing is the secret of success, then a senior BA manager writing a book about BOAC in 1975 must be a masterclass in bad timing.
In 1975 the BEA/BOAC merger was clearly seen to be a disaster. David Nicholson had failed to give any sense of vision or common purpose to the staff of the two merging companies, which had dissolved into factionalism and in-fighting. In the subsequent repeated attempts to end this almost all the senior managers went (including Mr Bray, I think) and any mention of BOAC and BEA was very strongly discouraged.
It didn't work and the chaos continued throughout the 1970s until a desperate Margaret Thatcher bought in the biggest bully she could find, and John King finally imposed a single structure and some sense of vision and purpose to the BA we now know.
Not surprising, then, that Mr Bray's work was not widely known and circulated in 1975.
By 1986 the situation was radically different. In the frenzy of the BA privatisation, BOAC had long been forgotten by all but a few. If the book was then reprinted I doubt there would have been much interest. The moment had passed. History had been rewritten and BOAC was now bad inefficient airline. Anything saying otherwise was out of step.