It's absolutely sufficient that the EU has not the least bit of an incentive to give Britain a better deal than remaining in the EU. The negotiating difficulty

It's absolutely sufficient that the EU has not the least bit of an incentive to give Britain a better deal than remaining in the EU. The negotiating difficulty for the EU is that it's only in control of part of the real deal for the UK. One other big part is how well the UK can develop its relationships with the rest of the world if it's no longer constrained by things like the customs union. A third part, perhaps more easily overlooked, is how well the UK can develop its internal economy without the constraints of EU regulations. Right now, EU regulations affect not just goods and services exported to the EU, and not even just goods and services exported at all, but also a lot of things that are bought and sold within the UK itself. These other factors mean that for almost any area of importance in the coming negotiations, there will be some limit beyond which the UK is better off taking its ball and going home, and then playing with the rest of the world instead of the EU tomorrow, purely on economic grounds. Of course, it's going to be extremely difficult for either the EU or the UK to work out exactly where those limits are, which is surely going to be a fundamental difficulty in those coming negotiations.