Until ARM was sold to SoftBank it was the primary British high tech computer firm. Little known fact - ARM is one of the very few European tech firms that incentivised its employees with equity sharing. It was and still is standard for firms outside the USA to keep all equity for the founders and investors, with none for early employees.
It's hard to know to what extent Acorn struggled to compete with Apple because of Europe-specific factors vs just bad luck: Acorn computers were very slow to get FCC regulatory approval in the USA, and they had a failure in their supply chain at a critical moment that pushed Brits towards buying US computers instead. They sold 300k machines but could only get 30k from the factories. There were a few other problems that stalled their expansion, and all this was taking place in the context of a country going through a major recession as Thatcher ended a government policy of employment targeting to bring inflation under control (inflation hit 22% in 1980). In the end they never recovered - US firms grew too strong and outcompeted them, despite initially stronger technology. Given that Acorn and ARM showed the same cycle of former employees becoming entrepreneurs and investors themselves, it's possible if things had worked out a bit differently for Acorn it could have been the "British Apple" and seeded a British computer industry competitive with Silicon Valley. But - it didn't.