Nothing about British society and culture is receptive to startups and entrepreneurs - it's an uphill struggle (both ways! in the snow!) to even get something simple started, let alone fully explore new ways of doing business.
The net result is that the entrepreneur has produced less value for the world than if they'd been able to build their idea in America, where there's far less friction in getting new ideas off the ground.
The US is (still) a unique and wonderful place for smart people to try to realize their potential. As a country in a global competition for talent, we should be doing everything in our power to attract and retain the smartest people in the world.
(UK emigrant)