There is some truth in all the points made, but there cannot be only one reason why it collapsed. They all played a part.
It takes a decade to build a new efficient factory, and to start volume production. Germany and Japan were forced to do that after WWII. Meanwhile, Britain used its old inefficient infrastructure and worn machines, during those 10 years to increase sales, but that all began to collapse, when it had to compete, in the international market, with the newly tooled up Germany and Japan. Japan only lost the first physical battle in their claimed 'hundred year economic war'.
If new machines being installed in British factories would last for 25 years, then they could only be depreciated at 4% PA. Overseas manufacturers used shorter, or zero depreciation schedules to build new factories. There was no incentive for the British to invest in building new factories in Britain, but overseas companies could lease-lend equipment, at low foreign depreciation rates, to their subsidiaries in Britain. The principle weapon, used for international economic warfare, is dynamically changing economic factors.
To compete in the international market requires a dynamically levelled playing field. Over the last 20 years in Australia, we have lost the last of the car industry. Depreciation rates have responded by falling from 25 years, down to 3 years, and for many things, can be written off the books in the year of purchase. That tells us something about the financial agility needed by a company, to survive in the competitive world market.
British production workers did not stop long for tea and biscuits during the day. If management had paid a fair living wage, and provided a safe workplace, the skilled workers would not have left the industry, and those who stayed would not have gone on strike. The inefficiency of the old industrial structure, caused management to underpay and exploit the workforce, in a failed attempt to survive. The 'British owned' factories could not be rebuilt economically, so the fate of the British owned industry was sealed.