England/Scotland border
In the last referendum, the counties just north of the border voted solidly 65+% to remain in the UK. There's not been a practical border there in three centuries, many of those people live on one side but work, or send their kids to school, on the other. You think they'd be happy if that became a hard border? How many of those communities will ask to have the border redrawn so that they can stay in the UK? How much opposition will the SNP raise to that? The equivalent border commission in Ireland in the 20s did not go smoothly. Solving a few customs issues in Ireland will be child's play compared to resolving that.
Biting the hand that feeds IT
