Its been alluded to before, but one of the biggest mistakes was the inability to get management in British Industry (not just the aircraft industry) to plan properly and make sense of production
Take an example of my mother. During the war she riveted together the majority of the starboard-side inner wing boxes for Spitfires built at Westlands. Her bridesmaid assembled the port-side boxes. My mother went off ill for three weeks (an ulcer). When she got back she was faced with a pile of boxes to build. No-one else had been detailed to take her place. There was a three week backlog of part-assembled Spitfires awaiting starboard wings, despite the urgency.
THAT mindset, THAT lack of planning resource was one of the major failings in UK industry