Here are just a few.
Every year exports out of Great Britain to non-E.U. nations literally double, while they decrease by half to E.U. states. In the meantime the European Commission regulates British industries so they don't out-compete the less productive nations within the Union. In fact, the E.U. trades with very few nations outside of Europe.
The European Union is at its heart an inward looking system that is designed to limit trade and commerce, not stimulate it. Add to it that many of the weaker European nations (i.e. Greece and Spain) require financial aid from dominant members (i.e. German, France and England) to stay solvent. This all makes sense to those who are engaged in a romance with wealth re-distribution, but clearly this is recipe for financial disaster. It doesn't take a psychic to see that first world European nations will be dragged down bailing out the second, and arguably third world members of their alliance.