Bankrupting ourselves fighting (rightly so!) the 2nd World War
Sharing technology - jet engine, all moving tailplane (rightly so - we were fighting on the same side as the USA, obviously)
Sharing technology - metal fatigue after the Comet 1 problems (rightly so - it was a moral issue which saved lives)
Concentrating on the development of bombers in the 2nd World War; we bought most of our transport aircraft from the USA so they had a lead after the War
Developing airliners which would land at the 'hot and high' airfields of the Empire/Commonwealth which were really too small whilst the USA assumed runways would be lengthened for larger aircraft such as the 747 (Boeing took an enormous risk with the 747, so designed it as a freight aircraft too - very sensible)
Government assuming missiles were the way forward in the late 1950s rather than aircraft
Prestige developments such as Concorde rather than mass market such as 747
Too complicated specifications - TSR2
Sir Stanley Hooker says the aircraft industry from 1955 - 1965 was mismanaged
Too many competing small aircraft manufacturers