Who cares if the millionaires did leave? I expect most of their money is held by shell companies in the Isle of Man, Guernsey, Bermuda, Turks and Caicos, BVI, and other tax havens that Britain hasn't got around to regulating yet.
The point, I argue, is not just the leaving aspect, which is obviously untrue. It's the fact that without taxing them, they tend to meddle in politics and turn a reasonably good society into a social and political chaos. By taxing them we, the society, exert a form of very necessary political control whilst also funding essential services that make life liveable for the individuals.