As another commenter already noted, the common market covers goods, people and services, including financial services. So in order to sell financial services, you need to be located in the EU. That just changed in the case of London, with obvious consequences:

— Ben Thompson (@BBCBenThompson) June 24, 2016

Sources at Morgan Stanley tell BBC it's already begun process of moving 2,000 London based investment banking staff to Dublin or Frankfurt