I suspect the thing that will ultimately trip up Britain's ambitions will be politics, not economics.  The EU is not just the common market, it is Europe's political response to a century of horrific warfare that left tens of millions dead and scores of cities in ruins.  Europeans are determined not to let nationalism break out again because they know the consequences.  So, beyond the balance books, they will want to make sure that no other member follows Britain out the door.  That means that they cannot agree to a deal that leaves Britain better off outside the union.