Eze Vidra, leader of Google Campus on what it takes to build a successful startup – London as a startup hub:

First, we have access to talent, with some of the top universities in the world based here, in addition to global talent that flocks to the city as a metropolis. Second, we have access to capital – there’s a huge concentration of wealth in London, and more capital is flowing into startups. Finally, we have ideas. While Europe was previously seen as the copycat centre for startups, more and more I’m seeing UK based startups who are disrupting their markets globally. London is a bit behind some of the leading startup hubs when it comes to density of network, but the momentum is playing in our favour, and it’s incredible to see how quickly it is improving.

Versus the best answer as voted by users on Quora: ‘Why do most successful startups come out of the USA?” – much more detailed in the answer itself; needs to be read along with all the other useful replies.

• Access to role models. 
• Access to funding. 
• Access to business infrastructure. 
• Access to distribution. 
• Access to monetization capabilities. 
• Access to talent. 
• Access to practiced management. 
• Access to production.
• Sheer serendipity

And interesting/somewhat related – FastCompany looks at whether there’s a link between median household income and startup success (see “Does wealth lead to innovation?“) Not particularly: