You are quite clearly misinformed what I actually takes in most European countries to get a banking license.
It's not quite the same thing as opening a corporate entity in, say, Delaware.
And as the EU made quite clear: Renting a mail box in Frankfurt and setting up a limited company won't quite do the trick.
https://www.bbc.com/news/business-40258449
So a few things from that article are important. The first is that a subsidiary already exists. The second is that some clearing already occurs in the US, so there's precedent for non-EU Euro clearing. And third, it looks like the European Central Bank has been trying to get clearing out of London anyways, but was stopped by the courts.
So it's not likely to be stopped since the US clears transaction, and even if it is there's already a subsidiary in France to pass it all through. There'll be some local job hits, but the sky is falling stuff is nonsense. Counties and businesses will still want to do business with UK and they will figure out how to do it very quickly.
Meanwhile they get to drop the EU common external tariff. That alone will be a significant economic benefit to UK.