Sorry I can't do links but found this from the Evening Standard:
"At least 400 UK-based individuals earn, or are capable of making, £10m a year.
But only 65 paid income tax, according to the latest figures obtained under the Freedom of Information Act. The rest use a battery of sophisticated but legal techniques to avoid paying.
It is not known exactly how much money is being lost to the taxpayer in this way but analysis by the Standard suggests it could be as much as £2bn a year"
Most very rich people should have a decent accountant and will wriggle out of paying any tax hikes. This whole argument seems rather hypothetical anyway.
It is a vote catcher for Gordon but won't raise any more revenue. And I can't see a lot of rich people high tailing out of the UK either.