As suggested already, the civil industry was killed by its slavish obedience to the demands of the two national airlines BOAC and BEA, which in turn reflected the incredibly short-sighted view of their place in the world; BOAC in particular thought, even in the early 70's, that its cabotage rights would continue until the end of time.
And, of course, both the civil and military manufacturers were at the whim of idiots in Goverment holding R&D purse strings; British civil servants with their notion that languid incompetence was and remains the best approach for an easy life; and British politicians who were always, then as now, stupid, only concerned with the next elections and frequently open to bribery from one or more interested parties to support or kill a project.