This might be a negotiating lever with the actual trade negotiators, who care about these things and know about such intricacies. But note that every deal with

This might be a negotiating lever with the actual trade negotiators, who care about these things and know about such intricacies. But note that every deal with the UK will also have to pass through all the national parliaments in the EU. And there (as in the general public) you won't find much love for banks in general and British investment banks in particular. When Philip Hammond talks about retaining access to "hedging" and "derivatives", to most people those are not things to safeguard but evil market speculation that should be eliminated. I have the feeling that (from the EU perspective) the opportunity to punish banks with a difficult Brexit will actually make negotiations for the UK harder, not easier.