There are so many things happening here, every day, every night. The Anglo-Saxon cultural origins of England (which are shared by North America) mean people regard individualism and independence a lot more favourably than elsewhere which creates an ideal climate for starting up your own company.
As a leading global city, people are drawn here from all around the World every day so there's an incredibly large talent pool here. By definition, everyone speaks English here which can make things easier. As one of the main financial centres of the World, obtaining financing here is a lot more easier than at many other places.
England being an EU member, you can enjoy the advantages of being located within the EU (hiring, easy access to the European market, etc.); at the same time, there's considerably less government regulation than in other EU countries so you would have to deal with a lot less red tape.