Dear PM, if Scots want closer links with Europe, why not? Let’s have a Scottish protocol

England never gets Scotland right. Last week the prime minister, Rishi Sunak, repeated Downing Street’s familiar gloat over another reverse for Scottish home rule. London’s supreme court dismissed the Scottish National party’s bid for an “advisory” plebiscite on whether to hold another independence referendum. Just go home, said Sunak, and run Scotland better. He seemed to think the SNP’s Nicola Sturgeon would apologise for wasting his time. He merely reinforced her party’s antipathy to London and all its doings.

The court’s decision was legally robust but politically inept. Sturgeon’s poll would have been purely advisory. The court appeared to be saying that independence was a forbidden subject to Scottish opinion, lending force to her claim that “the notion of the UK as a voluntary partnership of nations … is no longer a reality”. In the unlikely event of the Scots “advising” another referendum, then real questions might be asked as to what independence involved.

Why not ask them now? Sunak’s best policy would be to take the initiative. He should appoint a commission to ask what devolved powers does Scotland lack within the UK. Federal constitutions across Europe have exhaustive experience of “devo-max”. Scotland already controls its own health, education, transport and planning. It enjoys extensive fiscal discretion, notably over income tax. It could have more, as over corporate and sales taxes. But the quid pro quo would be the steady removal of the Barnett subsidy – £35bn last year – which underpinned a Scottish budget deficit that was 22% of GDP. It has since fallen to 12.3% of GDP, but if an independent Scotland were to seek admission to the EU, rules state this would have to reduce to 3% – a crippling adjustment.

Related: Thursday briefing: Nicola Sturgeon’s bid for a second Scottish referendum isn’t over … yet

European federalism takes many forms: Swiss, Spanish, German or Italian. All offer models and lessons and it is hard to see why Westminster is so dismissive of them. The instinct against home rule for UK nations recalls the fierce hostility to Irish devolution in the 19th century that led to total independence. It has been a long haul, but this year Ireland had the highest rise in GDP of all OECD countries. The UK is 38th. If I were a Scot looking to Dublin, I would find independence an attractive goal.

Maximising Scottish devolution must make sense. One issue is Europe, with EU membership overwhelmingly favoured by the Scots. The Northern Ireland protocol is now being fashioned to enable it to remain within the UK, but as a member of Europe’s single market. There is no reason in principle why Scotland should not follow suit. Things might get messy along Hadrian’s Wall, but there is a similar EU border between Norway and Sweden, and Germany and Switzerland. Where there’s a will there’s a way.

London should be proactive not reactive. Scotland is evenly divided on independence, which is a solid base for a federalist conversation. Sunak’s commission should ask simply what Scotland feels it shares with the rest of Britain and what further powers it wants for itself. It should be challenged on its deficit and fiscal autonomy. The debate should be on realities not legal niceties. Then the Scots can vote.

• Simon Jenkins is a Guardian columnist