One other factor that weighs heavily on entrepreneurship is the cost of land (and therefore the cost of living). The UK is cramped nation of 60 million people - space is in high demand, driving up the cost of land. The US is a vast nation that remains sparsely populated, even with 300 million people in it. The risk of starting a business (and possibly losing your home) is much more significant in the UK than US. For the more extreme contrast, compare with Japan (130 million in a country most mountainous that 75% is uninhabitable).
As a side note, land is more expensive in California than elsewhere in the US, approaching the value in the UK. In my experience, Texas with a reputation for cheap land, hard work and can-do attitude, is much more entrepreneurial than California. Silicon Valley has historically built an innovation cluster (compare with London's financial industry) and now has all the ingredients to make building new high-risk businesses much easier.