When a 62% Remain vote in Scotland met a 52% Leave vote across the UK, the Union of 1707 looked shaky to many. Nicola Sturgeon promptly said a second independence referendum was highly likely. But since then, somewhat to my surprise, support for Yes has looked pretty similar to 2014.

This has, so far, held even as it’s become clear Theresa May takes a hardline view of the Brexit vote. No doubt as a result, Sturgeon – who has no desire to lose a second referendum and really bury the issue – has tried to keep her options open. Her stated position is that her priority for now is to keep the whole UK within the single market. This would be the best possible Brexit outcome for Scotland and for the rest of the Union.

However, she has also spent plenty of time arguing Scotland could stay in the single market even while the rest of the UK left. Clearly, this ‘differentiated solution’ has a distinct political edge: Sturgeon has an interest in the UK being seen not to meet Scotland’s wishes. Still, the UK was always asymmetrical, and voters in Scotland voted decisively to remain. It is also fair to say Sturgeon has softened her position quite a bit from demanding Scottish EU membership post-Brexit. So taking it at face value, what would the differentiated solution entail?

The ‘differentiated solution’: Liechtenstein-in-Europe

The Scottish Government’s paper Scotland’s Place in Europe suggests Scotland could stay in both the EEA and the UK customs union. The document cites the principle of ‘parallel marketability’. This is the model Liechtenstein uses vis-à-vis Switzerland, while participating in the EEA. Because Liechtenstein has a customs union with Switzerland, it has to adopt Swiss technical standards and regulation in a wide range of areas. But the EEA also requires Liechtenstein to adopt its technical standards and regulation.

As a result, EEA and relevant Swiss law are now simultaneously applicable in Liechtenstein. Liechtenstein agreed this with Switzerland, and the EEA also agreed to accept the arrangement. Products meeting either Swiss or EEA standards can be sold in Liechtenstein, but EEA law trumps Swiss when dealing with the rest of the EEA. Liechenstein also had to create a Market Control and Surveillance Mechanism, so neither EEA nor Swiss rules were broken outside the principality.

The Swiss Federal Customs Administration collects duties on goods entering the whole customs area. It informs Liechtenstein’s Office for the National Economy when goods are destined for Liechtenstein. Where customs tariffs are different (some EEA goods attract tariffs in Switzerland, but not Liechtenstein), duties are reimbursed. Importers are informed of their obligations, including proofs of sales in Liechtenstein.

Why it wouldn’t work for Scotland

For Scotland, this presumably means HM Revenue and Customs would continue to collect duties for the whole UK. It would then inform a Scottish public body where requirements differed and when goods were destined for Scotland. By analogy with HMRC, the Scottish body might be a beefed-up Revenue Scotland.

The UK would also have to devolve much of employment, commercial and competition law, so Scotland could implement EEA law. Alternatively, the UK might confer a general power to implement EEA rules on Holyrood. But that could cause chaos in UK-wide law.

Where the UK’s rules didn’t mirror the EEA’s, new non-tariff barriers would arise, which poses a major problem. 64% of Scotland’s exports (excluding oil and gas) go to the rest of the UK, compared to 15% going to the rest of the EU. Even ignoring the fact that much of the oil and gas also goes to the rest of the UK, it is clear who Scotland’s main trading partner is. The Scottish Government would say the model keeps UK goods standards for UK trade. But Scotland is mainly a service-based economy. In fact, Scotland trades more in services with the rest of the UK than it does in goods. The position is reversed in trade with the rest of the EU.

So given that, what happens as UK and EEA employment law start to diverge? What about financial services regulation, especially when UK financial services are so intertwined? What if EEA competition law is tougher than the UK variety – what does that mean for a UK-wide company operating in Scotland?

The problem gets worse when you remember this is Sturgeon’s proposal in the event of a hard Brexit. The further the rest of the UK pulls away from the EU, the more acute the problem becomes. In Liechtenstein’s case, Switzerland adopts most of the acquis communautaire for goods anyway, which softens the dilemma. The UK may well copy a lot of EU regulation anyway, as it discovers the irritations of diverging from its main trading partner. But to the extent that it doesn’t, Scotland will pay a price.

Above all, Liechtenstein is a small and sovereign state. Legally, presumably the UK would need to join the EEA and then restrict its territorial application to Scotland. Scotland would then need powers to take part in the EEA Joint Committee and the EEA Council, appoint a judge to the EFTA Court, select a member of the EFTA Surveillance Authority College and so on. Spanish domestic politics on their own make it clear this won’t happen. But even if the politics could be resolved, the cases are fundamentally different. Liechtenstein has about 37,000 inhabitants. It is tiny, and therefore allowed to be anomalous. Scotland, on the other hand, would have the largest EFTA population in the EEA. It would also be far more closely bound to the rest of the UK than Liechtenstein ever was to Switzerland.

Liechtenstein’s single market model is not workable for Scotland, any more than its migration quotas set a precedent for the UK. Pursuing it undermines the overriding need for a sensible UK-wide deal.

On Brexit, powers reserved should be powers shared

With the potential and partial, but crucial, exception of Northern Ireland, the form of Brexit mostly needs to be settled UK-wide. But that doesn’t mean Scotland’s (or Northern Ireland’s) distinctive views should be ignored. The UK-wide settlement needs to be a compromise befitting a narrow vote and a territorial split.

In many ways, Scotland, Wales and Northern Ireland have federal relationships with the UK. Each has a legislature of its own as well as the UK Parliament. Powers reserved to the UK Parliament are specified in law. The courts can decide if devolved legislation exceeds devolved powers. Devolved powers aren’t meant to be changed unilaterally, though what that means is obviously disputed.

One key difference with federalism is the lack of formal protection for devolution. Unless Section 28(8) of the Scotland Act 1998 is read very radically indeed, no court would actually stop the UK Parliament legislating over devolved legislatures’ heads. But another is the lack of a mature federal political culture. Federalism usually involves shared rule as well as self-rule – an idea that a state’s nations or regions have a say in government at the centre. In a state large or diverse for federalism, central government has to govern with the territorial dimension in mind.

This is why many federations have indirectly elected second chambers. It’s why federations usually require some or all constituent territories to agree some or all constitutional changes. Modern federalism requires governments to work together. But at the moment, we ‘deal’ with problems between the UK’s nations by ignoring them or offering more powers. With Brexit, working out where to handle powers no longer exercised at EU level will matter a great deal. One of the many arguments for the EEA is that it reduces the need to create a framework for a UK single market which neither undermines devolved powers nor means more non-tariff barriers within the UK.

If the SNP wants to serve Scotland’s interests, however, it needs to recognise the UK can’t just devolve itself out of this problem. Scottish Labour, meanwhile, should push hard for UK Labour to back staying in the single market. But in some ways, it’s the Scottish Conservatives who bear the biggest responsibility.

After the referendum, Ruth Davidson said she’d prefer to stay in the single market post-Brexit. She has built her success on being an uncompromising voice for the Union of 1707. No one will ask her to row back on that. But Scottish unionism shouldn’t just be about keeping Scotland in the UK, but helping Scotland shape the UK. The Scottish Conservatives have political capital. Ruth Davidson is highly respected by her UK party. She has shown Tories can make headway in Scotland. She should use it to make Brexit respect Scotland’s interests, and work better for the whole UK.

What happens otherwise?

Some of the Tory right will say Scotland wouldn’t really leave over Brexit, hard or soft. Well, it might or it might not. Brexit complicates and sharpens the choice for Scotland, and the harder the Brexit the truer that will be. Scots would need to be prepared for a decades-long, probably very painful reorientation. It would be a far more radical form of independence than Alex Salmond proffered in 2014.

But wise unionists should want to reduce Scottish discomfort within the UK, not foment it. They should also beware of assuming Scotland would never jump. A Scotland where 45% voted to leave the Union is not one where the Union is secure. It is one in which the Union has much to prove. If Theresa May really cares for its ‘precious, precious bond’, she would do well to bear that in mind.