I just read an article that claims the opposite, ie that Brexit made it 30% _cheaper_ for Softbank, due to Yen vs Pound exchange rate.
"Brexit Made ARM Holdings A Bargain For Softbank"
http://www.forbes.com/sites/parmyolson/2016/07/18/arm-softba...
Accordingly if you look at the FTSE100 index, it's moved up about as much as the pound has dropped because a lot of the companies on it will profit from a weaker pound, while if you look at the FTSE250, which includes a large number of companies with income predominantly in pounds, it's down.
Some certainly faces currency risks too, but then the question is how much are you willing to spend to hedge?
It's highly unlikely most companies have sufficient earnings that they could have afforded to hedge against movements of this magnitude for anything but a short period. Certainly they can - and I'm sure some have - reduce the impact and give themselves time to adapt, but the cost of hedging against the effects of movements of this size and effects potentially lasting years would be prohibitive for almost anyone.
I do agree with you the entire Brexit situation makes it tempting to look elsewhere, though. Personally I do consulting, and I've started looking for US remote devops consulting gigs in preference to UK ones because it's suddenly a lot more attractive to get paid in dollars...