So many to choose from!
1957 Defence White Paper. So many promising aircraft and projects consigned to history. Reportedly the Saunders-Roe was a very viable contender for the contract for the West-German Luftwaffe eventually won by the F104G ('Widow Maker'). The only projects to survive were the Lightning, which although 'survived' could have been developed much further, and the ill-fated TSR-2.
Failure to properly rationalise the industry in the post-war period. Too many small manufacturers facing up to the 'big Americans' and therefore lacking industrial and financial clout. When the firms were rationalised, it was done poorly and inefficiently, leaving us with perhaps too few and just BAC (lately BAe) which has proved perhaps too big and powerful (in political circles at least...)
Wilson Govermnent Defence Cuts (mid 1960s). I could have just gone down the TSR-2 route with this one but that's a well trodden path so I thought I'd keep it broad. Granted the P1154 had its flaws, not least the conflicting requirements of the RAF and RN but to bin all that indigenously developed technology - such as Plenum Chamber Burning - and instead burn the cash on the most powerful, largest and yet slowest Phantoms in the world does seem somewhat mad. TSR-2 was a terrible shame but yet I can't help feeling that it wouldn't have quite matched up to the expectations. That said the way the cancellation was handled was nothing short of shameful and again we lost a lot of good technology in the process. Granted (again) the UK government couldn't have envisaged the protracted development of the F-111 and the associated cost-escalations, but the bigger problem was the 'capability gap' it left us with meaning the Vulcan had to soldier on for nearly 20 more years until the Tornado replaced it.
Scrapping of the 'conventional' carriers. Pretty simple this one. 'Proper' carriers with Phantoms (I never said they weren't good aircraft!) and Buccaneers and a very competent maritime AEW aircraft in the Gannet = a pretty good deterrent to Argentina even contemplating invading 'Las Malvinas'
Some others in brief:
Failure to see the Miles M-52 through to completion
Giving the Americans and the Soviets jet engines 'on a plate'
Lack of development of the Buccaneer (perhaps Mountbatten was right?)
Failure of De Havillands to engineer the first Comets properly, given that metal fatigue was a known phenomenon by this time - pilots of Hawker Typhoons whose tails parted company will testify to that!