Positive side: Less tech people = more money (which already has happened IMO, although maybe that's simply because accounting in big companies is in USD). Negative side: Less tech people...
EU techies have already started to move away. And some of the UK companies I've kept in touch with are really struggling to hire now. Personally, I'd always take a good office atmosphere/culture over more pay and a less skilled team.
As for non-EU techies, judging from one of our co-workers, getting a UK visa is horribly complicated. Don't think they'll make up the numbers fast.