Theresa May took a roundabout route on Monday to the meeting of the Joint Ministerial Committee. It includes the UK central government and all the devolved administrations, including the wee ones in the Channel Islands and the Isle of Man, so that their leaders can deal with matters of mutual interest face-to-face. It allows items on to their agendas that might not otherwise get there.

This meeting took place in Cardiff, where the Prime Minister travelled by way of Dublin. She felt a need to consult the Irish Taoiseach, Enda Kenny, in advance: of course the Republic of Ireland is not part of the UK, but in certain respects his wishes and intentions carried as much weight as any others she was going to hear later that day when she arrived in the Welsh capital. There indeed the JMC seems to have spent much of its time on a new Irish problem, leaving the Scottish delegation of Nicola Sturgeon and Mike Russell to fume at the lack of interest in what they were bringing to the table.

Nearly half-a-century ago, as the troubles in Ulster started to mount towards their murderous height, the dogma from the Conservative government of Edward Heath stated they were no concern of the Irish Government. They remained a purely internal matter for the UK and any mediation from Dublin (which was freely offered) would be unwelcome. Today we know that, without the mediation from Dublin which later led to and supported the Good Friday Agreement of 1998, Ulster might still be at war. We learned the hard way how close integration in mutual respect is the way for us all to live in peace and prosperity on our Atlantic archipelago.

But today barriers that have come down threaten to go up again as a result of Brexit. The claim from May and her ministers is that the Brits should “take back control”, by which they mean above all control of the borders, so that Europeans can no longer move freely among us. Because we live on islands they reckon, in their usual excess of optimism, that this can be achieved without too much trouble.

In any event these islands do contain one land frontier, running circuitously from Newry to Derry. It has always been porous. In the innocent old days it was a line across which pigs might be profitably smuggled, and later it offered easy escape routes for the IRA. Since it has never been effectively policed in the past, the chances of doing so now are remote. But its symbolism is stronger than its physical reality. Ireland is not today united, but it is more integrated than at any time since 1922. Republican Irishmen need no longer feel their country is a victim of British imperialism, so they have no motive to become terrorists. We tamper with this fragile settlement at our peril.

Yet, one way or another, Brexit must tamper with it. If the aim is to close the UK’s border at Dover, then logically it ought to be closed at Newry and Derry too. May and Kenny agree this would be in the highest degree undesirable, quite apart from impossible. In that case, what is to stop Poles flying from Warsaw to Dublin, making their way to Larne, getting a ferry to Stranraer and entering the British labour market – legally if they can, illegally if they must? The answer is nothing. Now May and Kenny must think of something.

Since at the moment nobody else seems much interested in listening to the Scots about our own concerns, this may be a point at which we can make our presence felt in a different way. Scotland should have higher immigration, not for the sake of political correctness but because we require it if we are to grow our sluggish economy. Growth involves three factors of production: technology, capital and labour. We have plenty of the first two, but not enough of the third. Any modest prosperity we have attained in the last couple of decades has therefore in part depended on immigration, on importing useful workers from abroad.

In the census of 1981, 1991 and 2001 our population fell. It was at one time forecast to go below five million in the census of 2011: think how depressing that would have been, to live in a nation so visibly in decline. But the first decade of the new millennium saw a surge of immigration, and there are today 5.3 million Scots, more than ever before. I think it’s great for us to have so many Pakistani Scots and Polish Scots. We cannot in fact do without them, because our own native population is shrinking. Its only growing cohort is the oldest one. The UK Government claims Scottish interests are being taken into account in the Brexit process. But what happens when Scotland needs immigration while England rejects immigration? Why, what a surprise! The English preference prevails.

If I were a member of the Scottish Government, I would be urging the First Minister to exploit these differing economic conditions and the demands that legitimately flow from them. It is not as if in Scotland we lack the room for immigrants, and we could offer them incentives to move to the outlying parts of the country, like the Syrians who have settled on Bute. We should also ease the stipulation that immigrants must earn a certain level of wages before they can gain the status of a resident. At the moment it is set at £20,300 a year, which I would have thought quite ample for Scotland, though only about half the average income for the south-east of England. And we should try harder to let foreign students stay on and work in Scotland after their graduation.

These ideas might strain relations between the governments in Edinburgh and London, but that is in the post anyway. We would have different policies on immigration in two countries that share a second border in the archipelago, though not a state border — the one running between Berwick and Carlisle. This would seem bound to lead to stricter controls, perhaps even to passport checks at the crossing points. It is hard to imagine, however, that these would be imposed from our side. Indeed the responsible minister in the Scottish Government, Humza Yousaf, insists they would not. No, they would need to come on the English side, ordered by the Government in London. Who would then be the separatists?

Obviously I’m taking my argument to its logical extreme, though this is often a useful procedure to expose flaws in the contrary argument. We live in a more complicated world, one above all of insecurity. Those who feel themselves its victims naturally take refuge in fear of foreigners and resistance to change generally. It is too easy and cynical for politicians to offer their simple solutions to these simple minds, as May does with Brexit. Brexit does not offer simple solutions. The Prime Minister should be more honest about the scale and complexity of managing immigration to the UK.