So lets be pragmatic then, all the people earning over a milion will not leave the country so stop being disingenuous. A number will choose to leave and what we should have a tax policy to suit them when they are so small a percentage of the population of this country? That certainly taxes us back to feudalism.
Those that go will sell all their assets here so revinue will be raised and tax paid on profits, as well as those who purchase their assets. If they choose not to sell assets they will still be liable for some revenue even if only council tax.
Might have escaped your notice but firms like HSBC et al, are far smaller than they used to be so the number of employees reduced would be comparable to the health service cuts, or cuts in local council staff. They were seen as expendable so why not the thousands of people who are involved in nothing more than customer service, managing other peoples money, speculating on the stock market. Like nurses and ambulance techs, teaching assistants, bin men etc etc they do not actually make anything.
Pragmatically speaking any government would then have to make decisions on trident, spending on building up manufacturing, investing in sme's, stop throwing money at the banks under the guise of quantitative easing.
Pragmatically speaking this version of capitalism is bankrupt, not sustainable, dead parrot gov. So your idea of keeping things as they are to pander to the few who have the highest earnings is anything but bankrupt unless you like your society riven by poverty and need.