Brunel: every company you mentioned there, and a hundred others I could mention, suffered from chronic mismanagement.
Let's take a few examples. British Leyland (aka Austin Morris and half a dozen other names) were making cars that by comparison with foreign cars, were old fashioned and not very good. Datsuns (aka Nissan) made far better cars, even if they also rusted very quickly. But Datsun and the foreign companies understood what people wanted: they came standard issue with the things people wanted. Car Stereos and the like. British motor engineering simply stood still. The much vaunted Mini Metro still used a derivative of the ancient "A" type engine. It rusted overnight. Even Rover, in its latter days, owed much of its sales to using Honda engines and some even used Peugeot engines. But did you know that we produce just as many cars today as we ever did? Just for other countries.......
One reason is, it is easier to fire a worker in Britain than anywhere else in Europe. That and huge tax breaks.
In my opinion, making things for multis or other nations is no way to run an economy. You just end up as a sweatshop.
Look at another British innovation, the Harrier. For a long time, we stopped making them and buy them back from the USA as the GR7, albeit with a much updated weapons system.
Almost all of our utilities are in foreign hands. The result? The highest energy costs in Europe. The energy companies apparently call this place "Fantasy Island", because they can charge what they like. High energy bills hits everyone, hence, everything costs more.
I find it so ironic that people believe that Thatcher and co "stood up for Britain" whilst they and subsequent Governments actually sold Britain off.
Service industries will never employ as many people as industry did, and they actually create a corporate mindset that is not British. Have you read Jeff Schmidt's book, "Disciplined Minds"? In it he argues that today's managers are the result of a stifling education system that crushes individualism and imagination (and morality) and in return pays its adherents enough to make them put up with this. In short, these are obedient people without a thought in their head but the goals of whichever business they work for. They do not think beyond corporate jargon and bullshit and would swear that black is actually white, if their corporation says so. They are rewarded not for their acumen or even results (look at the bonuses our bankers get, despite stealing £1 Trillion from the British public), but for their loyalty.
Compare these sort- and they're absolutely everywhere- with the likes of British firms during the 30's and 40's. The latter had a "can do" attitude and the former have a "sorry sir, I can't do that for you today" attitude.
This sort of management style set in during the late 70's at least. And accountants have more than their share of blame. The British motorcycle industry, for example, had dozens and dozens of makes, yet, on the insistence by accountants and useless management, did not innovate. A tiny 50cc semi automatic step through motorcycle by Honda started the wave that brought the British motorcycle industry crashing down. After all, if you wanted cheap transport to work, which would you prefer? An unreliable, 650 cc monster that could easily break your leg kickstarting it, and which leaked oil with depressing regularity, or a little bike that always started 1st kick, did 50 Mpg and was superbly reliable?
The British worker wasn't to blame, nor where the designers. It was management. We still have this problem now, because there's a belief that to be a manager, one must just be able to manage. It "doesn't matter" whether you're selling shoes or running a printing press. Most likely, everyone has suffered at the hands of such managers.