The EU’s existing financial regulation relationships with countries such as Norway, Iceland, Lichtenstein and Switzerland paint a picture of how the UK could evolve it's relationship with Europe.
Europe wants to trade with Norway and Switzerland, neither of which belong to the EU. So it allows Norway’s banks to passport under agreements in the European Economic Area, as do banks from Iceland and Lichtenstein. Switzerland is also completely separate from Europe, but has bilateral agreements to passport their financial services across the continent. This works to the benefit of European countries because Norway has the largest state-owned investment fund in the world, while Switzerland manages the assets of many of the world’s wealthiest individuals. This brings increased liquidity and investment to Europe.