Why would they move? Sure things are getting less centralized just like big financials are slowly moving from NY, but the heart of the city has no reason to move.
Firms move. Then people move. Check out MiFID2 (https://www.investopedia.com/terms/m/mifid-ii.asp). Firms used to be able to satisfy ~~German~~ EU regulations to trade on Eurex, German futures exchange, with HQs in London. Not so after Brexit leaves the UK no longer in the EU.
Yes mifid was implemented 3 years ago. How many firms moved because of it? zero.
I opened an office in London for a trading firm... So at least one?