Joking, they won't.
With an apology for the grabby title, here are some bits and pieces on the myth that if we tax the rich they'll leave. And by rich I mean really rich-not just Mumsnet 'we have three holidays a year but one of them is in the UK' rich (which is still pretty rich in my book), but 'we have a swimming pool in our London house' rich.
The UK is the only country in Europe apart from Switzerland which doesn't tax the income of non-domiciled residents; that's people who own houses here, and live here most of the time, but aren't registered as living here. If we taxed them in the same way we tax residents, (like Spain, Germany, France do) the yearly revenue would equal the current NHS budget. Even if some of them did leave, it would still be worth it. And it's hard to see what they're adding to this country as it is.
There's also very very strong evidence that UK nationals and permanent residents are unlikely to leave if we increase taxes. Cristobal Young 's research is really interesting. He uses US data, which is a really strong set to draw from. The US has freedom of movement across the states and independent tax structures, so according to the theory of 'millionaire flight' all the wealthiest people should just move to the lowest-taxing state. In fact they tend to stay in states like New York and California, which have relatively high taxes, because these are the places where they want to be.
I'm not sure how big an issue this is for most people but it's something I've come across a bit recently so wanted to talk about it tonight. Vote Labour, obviously.