However, looking at the history of tax rises and the competitive landscape, people don't move.
Broken down:
UK top rate tax was raised from 40% to 50% two years ago. They said people would leave. No-one did. Actually, more wealthy from BRIC nations came to the UK
The US is the only competitive English speaking country for people from the UK to move to. And I'm sorry to say that US taxation is horrendous and massively uncompetitive.
So while capital flight is a risk, it is improbable.