There is no doubt that the UK’s international reputation would take a hit. Would other powers, friendly as well as hostile, ask how a diminished UK could keep its permanent seat on the UN Security Council? Once a beacon of stability, a country that had turned its back on its near abroad and then fallen apart would not walk tall in international counsels.
Emotionally, the shock would be profound. The integration of 300 years could not be sawn through without huge pain, on both sides. If leaving the EU was like a parting from an extended family of second cousins, this would feel like the disintegration of the ancestral home. Over 800,000 people who were born in Scotland live in England and close to 500,000 people born in England live in Scotland. For many of them, separation would force a potentially deeply painful reassessment of national belonging.
Those without personal connections to Scotland might shrug this off as the unfortunate consequence of a decision taken by the people of Scotland; that’s where the blame would lie. But surely over that would be a shroud of failure, that something that had endured so long through so much adversity had come apart, that even those so close to England had chosen to walk a different path.
Nor would it necessarily stop there. For Northern Ireland, a UK shorn of Scotland would hardly make continued adherence to the union more attractive. The secession of Scotland would likely accelerate the already evident trend of increasing support for reunification.