Is this really surprising? London picks up all the advantages of the UK (legal system is sound, eg in contract law, native speakers of English, the lingua franca of business, strong university tradition) and adds in its special sauce: access to the City, global-tier culture, excellent public transport (I know we moan about it, but it really is excellent), and a timezone location that easily serves the US and Asia markets.
The awkwardness for founders in London is that when they want to IPO, London doesnt have nearly as deep a pool of capital as the US, so they are potentially leaving a lot of money on the table.
Is deep enough pool of capital for the vast majority of businesses.
Its cultural diversity is a plus for most people (other than people like DHH).
The big problem with London is that it is very, very expensive.
Startups might thrive there, but business investment in England (particularly in mature businesses) has not exactly been lively ever since Brexit. I can't recall the last time I heard someone talking favorably about investing in England, or at all, really.