If only it were a joke Petite!
Personally I think DC is a boy in a man's world. Our approach at the moment seems to be if you don't let us play the way we want to then we'll take our ball home. That's never a mature attitude.
It may well be that under DC we end up going from the UK as a core member of the EU to England billy no mates.
The only silver lining is that he's said several times that the referendum would be offered 'under a conservative government' and that at the moment looks unlikely.
Language, inertia and access to all the facilities of London will play a big part in keeping them here.
I'm sorry but English is the most commonly spoken language and the defacto second language in most of the world. I travel a lot for my work and language isn't a big deal. Any professional level job, the employee will be able to speak practically perfect English. Hell, I have a few dutch colleagues who can speak English better than most Brits!
And you are banking on inertia to keep big business here??? Seriously?
London is the financial capital of the world because of our timezone (we're slap bang in the middle), our friendly laws AND free access to the EU markets. If we removed that we'd see banks all wanting to establish a new European HQ so they can continue to trade in Euros. Their timezone is only +1 so no biggie. And English isn't a big deal either.
Lastly we don't need many big businesses to go or many jobs to be affected by import duty before we as a nation have a flipping big problem. Even using your 10%-20% yardstick. Can you imagine the impact to our economy if we lost 10% of our employment or tax revenues?