Here's the thing. UK's possible exit from the EU could cause an unprecedented shock that's hard to predict, possibly with global consequences.
The question these days is if England would leave the Euro'16 or the EU first. In the former case, it'll be only the football fans who'd suffer - for a while. But that wouldn't be anything unusual: England has often exited big championships early, despite always featuring among the favorites to win trophies. Option 2 would be more damaging, and could cause ripples across the whole world.