10% of British banks maybe ...
All the European banks (BNPP, DB, SG, DB, ING ...) moved to London City based trading HQ during the 90s for tax reason because they could also free trade to EU from there.
Now they anticipate that within a 3 year, this "opportunity" will be gone. They've already prepared plans to withdraw from the city. You know that banks don't like incertainties, do you.
So earlier those bank moved their activities from Amsterdam, Brussel, Frankfurt or Paris to the City ... but thanks to brexit now they are on the go for a rellocation to some EU stable area.
As UK is moving to be a tax heaven, I anticipate that within a 5 year, UK will be on EU grey list that will only help to perform shadow banking activities but from which will not help to perform the core banking duties : thus, little money for UK's treasury & more money for the EU countries.
By the way, the rellocation of the EU financial&economic institutions in UK will be the first to kiss good buy. EBA will be the very first ...
I personally don't care of "project fear" or "rule britania". What I see is a stupid decision from an economic point of view for UK citizen, but a great decision for EU politics & economy on a middle term basis. Simply speaking, there has always been a gap between the continent and UK (think Yougoslavia, Irak War or even Syria ...). UK has prevented the creation of a EU international policy and of a unified intervention army to cool down hot spot that endanger our liberty. Now UK will have to solve their puzzle on their own : Beeing a US puppet, Divorce with Scotland, Tax haven or not, Royal Familly & al.
By the way, I do hope that in France the next president will tear appart the Touquet traitee and ask the UK gov to perform their duty. You want imigrants ? Go and handle them. I don't see why FR should take responsability of cleaning a neighbour's drain.
As a global consequence, I also anticipate that the idea of EU Federation will be put on the table and that there might be room for citizen acceptance after the current period of nationalism backdraft is gone.
From a bad thing always come good things.