London's basically great for:

  1. Startups
  2. Scaling/Growth
  3. Existing company expanding from USA->Europe or Europe->USA

For start-ups/high-growth you get a pretty good business environment (legal/corporate and day-to-day running a company is reasonably straightforward), employees with skills and the employment law is pretty flexible, there's plenty of finance around.

For Growth and USA companies that are expanding into Europe then London/UK is somewhat easier to understand than other European geo's. There's an aligned language and there's more cultural touchpoints - it's kind of a mid-point culturally/institutionally between Europe/USA.

For funding/driving to late-stage and IPO there just isn't anywhere like the USA. But everyone has to expand to North America eventually, so it can be fixed at that point. Also, PE funding has changed a lot in the last 10 years, so if you're not on the 1-in-1000 rocketships there's a more steady 'good' path which is totally viable.