I was replying regarding why London rent is high in general. SF/The Bay seems to be disproportionately tech-based jobs. London is a more generic mixture, including finance, arts, entertainment, services, insurance, tourism and so on.
Though specifically tech: offices are in central London - Google at King's Cross, Apple at Battersea, Facebook near Oxford St. Old Street and Hoxton for start ups. And of course fin/fintech in the City, Canary Wharf, and the West End. That's 10s of thousands (if not 100s) of developers concentrated in around London.
Microsoft in Reading 20 miles outside London is a bit of an outlier.
No one drives to any of those jobs. And the talent pool for those jobs is centred around London and London transport.
And then if you want to raise VC you need to get into the City or Canary Wharf for meetings - they won't go to you as you need their money. And then they'll be interested in 'where are you set up, and where will you get your talent from and grow?' - I can't recall any significant tech company that's inside the M25 and not in Central London.
The "Microsoft UK in Reading" equivalent towns in any other direction than the M4 would be something like Maidstone (Kent, SE), Bishop's Stortford (NE), Milton Keynes (NW), Crawley (S). None of those have any significant tech companies. Weirdly, Guildford (SW) does have a tech scene partly because some gaming companies set up there in the 1980s and 90s.