You're missing the point: the institutions that are headquartered there conduct a huge volume of Euro-denominated derivatives trading, which becomes impossible

You're missing the point: the institutions that are headquartered there conduct a huge volume of Euro-denominated derivatives trading, which becomes impossible if the City loses its license to do so, and it's hard to see them retaining it if the UK leaves the EU. So those banks would have to relocate their investment arms to somewhere in the EU. That's got to be worth several points off the UK's GDP. Instant recession -- and it's not recoverable.