While UK banks aren’t currently affected by the Single Supervisory and Resolution Mechanisms of the banking union, they do need to comply with the Single Rulebook. Incidentally, the European Banking Authority plays an important role in this regard. Not only does the EBA ensure a harmonised interpretation of the rules, it also promotes interaction and dialogue on supervisory practices in the member states.
And as I’m sure you all know, it is currently headquartered in London. So if the UK voted to leave the EU, the EBA wouldn’t only have to find a new home; it would also have to rebuild the prudential cooperation with the UK from scratch. So a “leave” vote throws up a vast number of planning issues and raises considerable uncertainty.