It is in their interest; however the political integrity of the EU is more in their interest and the two are at odds with each other. Further, it isn't about is

It is in their interest; however the political integrity of the EU is more in their interest and the two are at odds with each other. Further, it isn't about isolation, rather the nature of the deal. The UK must trade with the EU at least in part because the current account deficit implies that we provide value add services: the initial value of which needs to be imported.