It took a long time for Britain to realise that its place in the world had changed massively post-WWII, and the attempts to produce indigenous weapon systems continued for over 20 years while this realisation took root.
It's only natural for a world power to take time to adjust its thinking, and for those in power and those trying to provide them with military hardware to continue on what, with the benefit of hindsight, turns out not to have been the best way of doing things.
Given that some of this happened when we were trying to recover from the massive spending required during wartime and pay back the lend-lease loans I can quite believe that what was seen as necessary to protect the nation, no matter how misguided when viewed from the present, took up a significant amount of scarce government money. It's very hard to believe your suggestions about the attempt to turn the swords into ploughshares either, all of our indigenous civil aircraft projects ended up with relatively low sales and few economies of scale, and Black Arrow managed to launch one single satellite and cost about £9m to do it, which is a paltry sum for the late 1960s.
No, I just don't buy it, and the evidence, as others have pointed out, can be found at the PRO in Kew if you want to devote some time to research it.