Scottish independence would be geographically dramatic for the UK: it would lose just under a third – 32 per cent – of its land mass. Yet the rest of the UK (rUK) would lose much, much more.

“Nearly all constitutional lawyers agree that Scotland, if it chose independence, would be a new state”, says Alan Boyle, Professor of Public International Law at Edinburgh University, “while the rest of the UK would be the successor.”

Thus the rUK would automatically retain its seat on the UN Security Council – the most obvious legacy of Britain’s past pre-eminence as one of the victors of the second world war. It is the largest platform on which it can, as a middle-sized, nuclear power, “punch above its weight” in the world.

But, says Vernon Bogdanor, Emeritus Professor of Politics and Government at Oxford university, “though the exit of Scotland wouldn’t immediately threaten the UN Security Council membership, other countries might argue that a diminished UK shouldn’t be a member; especially where other countries, much larger – India, Brazil, Germany – are not members”.

The issue highlights the dilemma facing England (with 92 per cent of the UK’s population). Its permanent membership of the Security Council is an anomaly resented by much larger states – the UK has one-third the population of Brazil, one 20th of that of India – but also by fellow EU members who want a common EU seat. The loss of Scotland would help that argument.

The second international forum in which Britain punches hard is Nato, of which it was a founder member. The loss of Scotland would be problematic for the rUK as the Scottish government has made clear it would demand the removal of Britain’s nuclear submarine base from Faslane on the River Clyde.

The submarine force is the totality of the rUK nuclear deterrent; no other base is equipped to service the four Vanguard nuclear-armed submarines and the 200-plus Trident submarine-launched ballistic missiles: an alternative would cost around £5bn and require a decade to build.

The rUK could seek an agreement with Scotland to continue to use the base: though the nationalists’ detestation of nuclear weapons would make that hard. Hugh Chalmers, a research analyst at the Royal United Services Institute, says that, while there are several examples of naval bases of one country on the sovereign territory of another, “this would be unique – in that it would be a nuclear force, and it’s likely that the rUK would wish to have it on their territory, especially as the stated Scots government position is one of opposition to nuclear weapons”.

Scotland had been crucial in the Cold War because of its position: its sole remaining RAF station (after the closure of Leuchars, in Fife) is Lossiemouth, on the Moray Firth in the North East, one of the UK’s largest bases of its Tornado fighter jets: the nationalists have said they would wish to keep the base, possibly operating it jointly with the rUK.

An independent Scots government committed to the nationalist programme would – at least in the short term – disrupt and weaken the UK defence posture at a time when Nato is dealing with a perceived threat from Russia.

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