City - '10%' is the latest chatter. No one of course knows what it would be yet. However, this has moved from a 'total destruction' on 24 June to about '10%' today. It is still a moving target.
And of course all the big banks already have operations in the EU countries. The operations are already being coordinated from across the Globe - go to any of the big bank website and you will see they have offices dotted across the EU, and the world.
If they could relocate they would have already - London has been a very expensive place to do business during the last few years (before deval).
They haven't go.
When we narrowly escaped joining the Euro, they said they will be gone - but apparently not.
HSBC has reaffirmed that it will remain HQ in London. We don't like the banks much (we tolerate them) but the EU countries dislike them even more. Do they really want to be HQ in Frankfurt?
I am afraid your 90% is way over the top and that is apparent to any informed commentator. You may be able to argue for a 25% case but I think that is still way OTT. 1/3 of the City operation serve the UK domestic market - that is no going anywhere. The other 1/3 serves NA and RoW - no particular reason to go to the over regulated Eurozone (there is only 1 country Denmark that is outside the EZ and they are definitely not going to go there) and 1/3 to the EU.
So, you will have to explain to me why this is 'blatantly obvious' that they cannot wait to rush for the exit.