London, being the financial capital of the UK and arguably the world, gets to remain a member of the EU. The fees are paid by the financial sector, and the UK's MEP's are placed under mayoral control. London abides to the standard EU regulations, and gets single market access. People living in are given some sort of passport or identification, and the rest of England gets to take back control or whatever. Similar arrangements can be made in Scotland and NI.
Just a rough idea, but adaptations can be made? I think this would be a pleasing arrangement to all parties no?
What should I do?