I just read today that the UK government had earmarked a plan to fund £300 million in research grants across universities in the UK, but not even 40% of those funds were allocated before the govt shut down the programme. That imo is a effectively a dearth in innovation capital.
Companies don't get hoovered up against their will. The fact of the matter is that moving to the US significantly increases your valuation, even if most of your activity happens elsewhere. If the UK wanted companies to stay, they should make it easier for companies and investors, both domestic and especially foreign, to invest within the country.
Case in point, you can start a factory in less than a year in the US (imo, the fastest in the world, even in these shitty times). That setup would easily take 3-5 years elsewhere.