I actually find it encouraging that there are still some real, principled nationalists is Britain who believe in real, hard borders and are not interested in some kind of apparent Brexit which leaves the UK as member in all but title of the EU.
Britain might be in a more flexible position than it gives itself credit for. Dave Davis and Boris Johnson are assuming the May compromise essentially will be a platform for the EU to gradually re-entangle the UK inextricably into the EU 'Customs Union', borderless community and Court of Justice with its principles of free movement of people, goods, services & relativistic legal dictates.
But it should have escaped no ones notice that the EU is not a monolith. Its is fracturing internally from its margins inwards under intense national, religious and moral tensions. The UK might be left as a sovereign nation again in the ruins of a disintegration of the EU regardless of the Brexit agreement.