most important treaties will be negotiated under new treaties, as it's in interest of both parties (people tend to forgot it's not in U.E. interest to have no deal at all). If there was a real threat, The City would have already left somewhere else, and it's not the case. Actually, it's pretty obvious that U.E. makes everything they can to impeach the Brexit that's why I tend to think the real threat is more for U.E. (as an institution) than for UK.