The competition isn't between London and Paris. It is about which EU cities will take the most business from London due to potential limits on market access.

It's not a question if banks will move some portion of staff out of London, but how many and where they go.

London does have more than Paris, but Paris and Frankfurt are the only two other EU cities that consistently get listed amongst the top global financial centres.

What exactly that means in hard cash depends on what you want to count and how, and so the rankings vary greatly depending on who you ask.