Consider that Azure, AWS, Google Cloud Platform, etc. are all huge in the UK, yet most of the work on them is taking place in the US. Ditto for Oracle, Office 365, the Google Suite and even consumer stuff like Craigslist, Reddit or Apple's services (that the UK never home produced something like Reddit continues to boggle my mind). Why pay for the best people in the industry to work in the UK when we can just stand on the shoulders of work being done in the US?
If you look at industries where the UK does take an innovative role like finance or bioinformatics, the salaries seem a lot more competitive, though are still not at US levels due to the UK's lower cost of living I guess.