[scholia]

Not sure why there's such a concentration on San Francisco: it's a much smaller place than London (10m inhabitants).

As a Londoner, I've visited tech companies in San Francisco, but also in San Jose, Santa Clara, Palo Alto, Mountain View, Cupertino, Sunnyvale and various other places. They're not all the same.

However, in the UK, a lot of tech companies are similarly spread out around the M4 corridor. Wikipedia says: "this part of the M4 Corridor is sometimes described as England's Silicon Valley" and includes "Slough, Windsor, Maidenhead, Reading, Bracknell and Newbury". https://en.wikipedia.org/wiki/M4_corridor

That seems to me to be a more useful comparison....