Mayor Boris Johnson says bonuses tax 'threatens London'‏

Looks like the issue of capital flight will continue to rumble on for a time yet.
Considering the actions of France and USA, I don't think you can properly call these unilateral actions.
"Mr Johnson said: "The bankers helped to plunge Britain into a terrible recession for which we are all paying the price and it was their behaviour that made recent taxes a political inevitability." that's about right, but the problem is that these financial institutions and actors are so powerful that government's are running scared. The parallels with the striking workers is there for all to see, and the miners' strike in particular. In that case the right backed the government over the miners, now they back the bankers over the government.
I personally don't think it's as bad as all that, that the rich and talented will be flocking overseas to escape a penny or two more on their taxes, but it's a very important issue.

What about 'banking fuck up threatens London' eh?