In 1950, Great Britain was the world's second largest producer of automobiles after the United States. Today, there are no major British-owned car companies left, and the few remaining British nameplates (Mini, Jaguar, Rolls-Royce, Bentley) are owned by foreign companies. What went wrong?
There's a few factors for the decline of the British auto industry.
Starting in the 1950's there were many labor disputes and even factory strikes at British car factories. Obviously this caused British manufacturers to lose millions of Pounds due to lost production.
British cars weren't exactly good cars. They had horrible build quality, especially their electrics. This reputation of badly made cars spread and caused sales to decline.
In a desperate attempt to save these companies and save face for Britain, some companies were nationalized, but it backfired as they kept bleeding money. Ultimately buyouts to the private sector would cause companies to either go to foreign firms, or outright disappear.