I'd always understood that while European countries saw a lot of their heavy industry destroyed, and replaced it with new efficient plants paid for by Marshall plan money, the British ones largely survived, meaning the British had to continue postwar with outdated and worn out equpment, falling behind their rivals. Maybe that's wrong though.
That's basically the reason.
Of course it goes deeper as well.
The UK was the first country to industrialise and thus in many ways was left with an inadequate industrial infrastructure. We had all the prototypes.
The French, Germans, Americans etc, had the finished models.
Take railways for example. We built ours first, with the early narrow gauges. These are less efficient because you can't carry as much, as cheaply, with large locomotives on wide gauges. Everyone else, who industrialised later on, were able to invest in national network of wide gauge tracks.
The same sort of process can be seen in virtually every other industry.
We also had a serious management problem. The industrial revolution had been started by ambitious entrepreneuers with little or no formal training. They created dynamic family buninesses and then, as often as not, retired to their country estates, leaving their business to their Eton educated sons.
France, Germany, America et al, when confronted with an industrialsed Britain that was hugely wealthy and threatening to dominate them forever had to play catch-up, quickly. And they did that by planning carefully. Large firms, special management training programmes, professional bodies, often government support.
By 1945 the UK had a serious management defecit. Our firms were small, family owned, fragmentary compared to the large corporations in our competitors.
We had owners who were more interested in shooting pheasants than merging and growing their businesses.
We had managers who were not professionally trained in mangement, but had a wonderful classics education.
In fact, we had another problem. The only people who were trained in a profession involved in running business, were the accountants. This meant that when in came to business strategy, the people who tended to be regarded as 'those who knew best', were the bean counters.
Accountants are not well known for being aggressive, ambitious risk-takers. And during the decades they basically guided British business, we found ourselves consistently out-maneurvered in terms of international competition with foreign firms.