There is the Tier 1 (Entrepreneur) visa, though it's not at all straight-forward - you need £200k investment, or £50k under conditions that I'm not sure it's possible to actually meet.
£200k is not a very huge amount of money by London standards. Maybe fully loaded cost of 2 senior developers for a year, if you get creative making it stretch.
Any threshold below that would effectively be a open doors situation.
I just pointed it out since you used 4 US companies as examples, and I thought you could be implying that it could be easier to start a company there for a foreigner.