On March 2, 2016, Mark Boleat, the Chairman of the City of London Corporation’s Policy and Resources Committee delivered a speech that merits to be quoted as length because it provides a detailed and unambiguous answer to this question:
By Mehreen Khan, The Telegraph, 19 Feb 2016
London attracts a vast number of investors and high income individuals from all corners of the world and in particular from the Middle East, Russia, and China. Its attraction consists exactly in being the most convenient access point to the entire European continent.
One should see London as the major hub of the complex network that stretches out throughout Europe and beyond. Amputating this hub from its constituent network will inevitably result in its contraction and faying away as Europe primary financial centre and the emergence of other major nodes vying to to replace it that are particularly well positioned to do so - one can think of Paris, Frankfurt, and Berlin in particular.