I moved from Silicon Valley to London -- a funny thing I haven't seen mentioned is that the tax rates on RSUs are absolutely awful (extra NI can push rates to nearly 60%).

That is a large disincentive for working in a tech company versus finance. Tech companies especially start-ups largely pay in stock which could be mispriced and you make more (or less) money than could be predicted. But versus finance paying pure cash, less (equity) risk, and a lower tax rate the incentives are clear.

HMRC I don't think should be underrated in their effects on answering the question -- "should I start my start-up in the US or the UK?"