Invasion of the actual UK is something that even Hitler never managed; in that case we launch the nukes and pray.

Far more relevant is what happens if a UK ally, vital or dispensible, gets invaded. Or yet another impending massacre of civilians, such as got us involved in Libya. As with the first and second world wars, are a huge number of people going to get killed or displaced in an unnecessary conflict?

Or if the UK manages to accidentally restart armed conflict in Northern Ireland; a Brexit that puts up customs posts there will get them blown up.

Would the segment of the UK that voted for Brexit in order to deport Poles cheerfully come to the defence of Poland?