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I know of many firms in the process of moving operations to the mainland.
A lot of firms were doing this in any case - London is one of the highest cost centres in the world, and certainly in the European time zone.
But yes, if Mifid is no longer in play for the UK, then there's less reason to have a branch in the UK, given the additional costs, restricted capital flows & probably increased regulatory inertia and bureaucracy.
I think that there is a big misconception about how much Brexit will hurt the banking sector. London is a global centre, but one of the big things that has helped this is the single market. A firm that is authorised by the FSA is counted as being auhorised in all member states. That disappearing will be a major issue. The threat of that happenning is already a major issue, I know of many firms in the process of moving operations to the mainland.