Practically speaking since war is unthinkable is would result in as much economic isolation as Europe can bear. It would be the end of NATO. Almost immediately there’s be European voices saying ‘What’s the real harm?’ and other appeasers. I think the political lash back would only last 5-10 years as parties opposed would find the only tool at hand - economic punishment - to be unsustainable. It would legitimise nationalistic sentiments in Europe even further. Britain would, naturally, talk of betrayal but not be able to make any resistance of any substance.