> taxing money leaving the country might be interesting way to approach the problem.
This would end very poorly because what the U.K. sorely needs is investment (to create new productive capacity). For example, Americans invested huge sums in North Sea oil and created an entire industry (before we destroyed it). Conversely, if you force people to keep wealth in the country then you just make things more expensive: they will bid up the price of property and the like. Nothing is added to the UK’s real economy by increasing the number of pounds flowing around in it - it’s only helpful if it’s invested. So what you actually want is tax breaks for foreign investment, but with some kind of ownership cap.