The City has one of, if not the world's greatest concentration of finance brainpower and capital, and has survived through hundreds of years of global turmoil as one of the world's leading financial centres. Do you really think these vested interests will just give up that lead and move elsewhere?
It became global leading when it happened to be the navel of the nascent Empire. With reducing global relevance of the United Kingdom and improvements in conditions for finance elsewhere there are fewer reasons for capital flows to be managed there.
Still may take a while and one may well live a fulfilling career in meantime, just saying it's far from a sure bet. Capital is not terribly sentimental.
