This idea is only slightly less delusional than the idea that Berlin and Paris will allow Europe financial affairs to be run from outside the Eu.
If we for one second assume that delusion to be in the realm of reality (very far fetched), yeah, sure, UK will be able to negotiate good terms with anyone. In fact, it can, in theory, negotiate better therms as the Eu cannot keep it on a short leash while it runs a "good terms or your assets get it" gambit.
The issue is that the idea of the London City after a Leave to be anything more than a glorified version of Virgin Islands corporate registry is exactly that - in the realm of "WTF are these guys smoking". Paris and Berlin will not allow that for a split second. That automatically removes any UK negotiating leverage in any trade negotiations.
When the City gets "nuked", some smaller hedge funds may survive (for a short time, then move). All larger financial entities including 99% of stock trade activity will move to Frankfurt within less than 2 years. With the relevant consequences for everything else (thank you Maggie for making the whole UK economy being wholly dependent on the City fortunes).