I am not sure why. Their would still be money to be made by banking in Britain. This sounds like doom and gloom offered by someone who just doesn’t want Britain to leave the Euro.
There, not their.
Britain never joined the euro or the eurozone; they still have the pound sterling. There is something really wrong if doing business with the City of London is contingent on the UK being in the European Union; very few profit-minded banks like the EU, and it’s only government toadies that want to stay with the EU.
They need the Brits to bailout Greece.