[throwaway2245]

London will likely be less attractive as a place to work for Europeans: since they cannot work without organising a visa and depending on the UK government (given also that the government said out loud they would put up EU residents' rights as stakes in their Brexit negotiations);

and likely less attractive as a place for to non-Europeans to work, since they can't get access to Europe via London.

Finding workers for your unicorn startup in the UK will be much more challenging.