The UK lost Deep Mind - which could have been OpenAI!! -- to Google. I think part of the issue is cultural - the level of ambition in the UK is just small compared to the US. Individual founders like Demis or Tom Blomfield may have it but recruiting enough talent with the ambition levels of early Palantir or OpenAI employees is so hard because there are so few. Instead, a lot of extremely smart people in the UK would rather get the 'safe' job at Google, or McKinsey than the 'this will never work but can you imagine how cool it would be if it did' job at a startup.
There are probably political reasons as well. Unfortunately the UK has not been well governed for 20 years or so, and hence economic outcomes as a whole have been abysmal.