I think the problem isn't so much being part of the EU as it is not being part of an isolated UK.
I must admit I'm in much the same boat - remaining in London is a lot less attractive if lots of the jobs are going to dry up (when companies relocate to the EU) - in that case, the US begins to look more attractive; it's an English-speaking country with lots of tech opportunities.
Why would companies leave? Is not there benefit to waiting and seeing how the dust settles? I have lived long enough to know that any change is always disruptive and after time passes the initial reactions also pass.
Financial companies have a large incentive to leave if the passport scheme, which allows them to sell their services in the EU, isn't retained in its current form. Other companies - if you do business with the EU, and the trade agreement isn't so favourable, then relocating again makes sense.
There is benefit in waiting, but companies will be planning for the worst-case scenarios.
I must admit I'm in much the same boat - remaining in London is a lot less attractive if lots of the jobs are going to dry up (when companies relocate to the EU) - in that case, the US begins to look more attractive; it's an English-speaking country with lots of tech opportunities.