Yes the EU was originally an economic union, but how can you have economic union without politcal union? Free trade, freedom of movement and less transaction costs might be well and good but how can you decide the detail of these decisions fairly without elected representatives?
For example, say we leave the EU tomorrow and try to join in a purely economic sense. Our diplomat goes to brussels and says we want x y and z. They reject our proposals and instead offer us something far weaker with taxes and limits on employment etc etc. We couldn't do anything to stop that because we'd have given up our diplomatic rights in the EU. This seems far less acceptable to me than to actually get a say in what happens.
Britain, France and Germany stand at a crossroads in the world. Our populations will never be big enough to compete with the likes of the USA, India and China, so we have a choice. Either we go it alone and fall by the wayside. Or we pool our resources and actually compete as a group, together.
Im not saying the eu doesnt have its problems. Yes its corrupt, yes its wasteful, yes the cap is awful, but if we leave completely we will be isolated in the world. It will not be glorious isolation, it will be lonely, weak and pointless.