On one hand they need some sort of integration to compete globally against the US and China in terms of security and trade, on the other hand, they are a wide range of economies, peoples and languages with a different set of fiscal and cultural goals.
Why do they need to be in a customs union to compete in terms of trade? It would probably be easier for European countries to make trade deals with the rest of the world if they could all do so independently instead of all having to do so through a single common trade policy.
What do you mean in terms of security? Does the EU have any substantial common military forces?
Would it help if they did?
So, what you are saying is that if there were no common currency, Britain could trade through Spain and through there trade with Kazakhstan that has lower duties on Spanish goods than British goods? That's a good point but it seems more complicated than a more concentrated approach.