Exodus? What exodus? With America now as she is, where would the refugee rich go?
"Oh, lots of places have lower tax rates." I do not doubt that taxes are low in the Congo. But these people want a certain lifestyle. Even more so, their wives and children, especially their daughters, do.
They can get it in precisely two cities on earth, one of which is in the United Kingdom while the other is in the United States, a country to which many of these Russians and Arabs, in particular, will already have been refused entry, or else they would already be in New York rather than in London.
London may be stuck with them. But they are stuck with London. Therefore, they are stuck with whatever tax regime we choose to impose. They have nowhere else to go.