There is no good reason why the United Kingdom should fall apart. But if it does – and sadly many Scots now seem to believe it will – it will not be because of what happened during the Scottish referendum, but because of what went wrong in the aftermath.

Historians will look back on months of tit-for-tat vanity misjudgments by our political leaders who, retaliating against a Scottish nationalist government pushing an exclusively “Scottish interest”, decided to elevate the “English interest”. They forgot that for the UK to survive, there needs to be a common UK interest that binds us together.

On the face of it, the case for the UK is stronger than a year ago. Not only did the SNP lose the referendum, but it abandoned its 70-year-long demand for a separate Scottish currency, and bought into being part of a UK-wide economic framework. Yet instead of finding a statesmanlike way of bringing Scotland back in, the government did the opposite. Its 7am morning-after response – English votes for English laws (Evel) – has come to mean only one thing, as confirmed by William Hague today: advancing the English interest at the expense of both Scotland and Britain.

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This would embed at the heart of our constitution two classes of elected representatives – the English who vote on everything and the Scots –and over time the Welsh and Northern Irish – allowed to vote on only some things. It would restrict the rights of Scottish MPs to vote in the Commons by excluding them under “consent motions” from budget votes on key tax issues. The Conservatives have acted with huge cynicism: it was they who proposed devolving all Scottish income tax to the Scottish parliament, which they now make the pretext for giving English MPs the power of veto over income tax rates.

Just when we needed reconciliation, the Conservatives have summarily repudiated the recommendations against Evel by the Smith commission that they appointed only four months ago. If we needed any confirmation of their potentially fatal mistake, it is that Scottish Nationalists who want to break the UK are Evel’s biggest cheer leaders.

Of course, many Tories and even some Liberal Democrats believe that the fundamental constitutional issue now is that Scottish MPs can vote on English-only laws, whereas English MPs cannot vote on Scottish-only laws. But this, the so-called West Lothian question that they think they have answered, is only a symptom. For Evel does nothing to grapple with the more basic problem the UK has always had to deal with: the population imbalance between the minorities – Scotland with only 8% of the UK population, Wales 5% and Northern Ireland 3% – and the majority, the 84% in England. If anyone’s interests are under threat, it is not England’s but those of Scotland, Northern Ireland and Wales that are permanently at risk of being outvoted. And so when one part of the union sends 533 members to parliament and the other parts only 59, 40 and 18 respectively – 117 in all – it is obvious that the rules needed to reassure the minorities are bound to be different from the rules needed to uphold the majority.

The UK is not unique in this. The same imbalances exist in the US and Germany. And Australia Belgium, Canada, Spain and Nigeria have faced the threat of dissolution over minority views being discounted. All have had to find special ways to recognise the needs of their minorities. But they understand that fairness of outcome cannot be secured by a blanket uniformity of provision. Unity can only be achieved by resisting uniformity – and by according some respect for minorities .

Paradoxically Evel, this hunt for perfect symmetry in an asymmetrical world, would make ours the first parliament in the world with two tiers of elected decision-makers. It would result in a constant nationalist refrain that there are now first-class English MPs and second-class Scots, the English who rule and the Scots there on sufferance – a recipe for constitutional chaos. Can you imagine Scotland volunteering to send MPs to Westminster indefinitely if it becomes clear that the UK government of the day owes its survival to an English majority who could veto the government’s tax policies even when that government can command a majority in the UK?

William Gladstone thought of offering the post-home rule Irish lesser rights than the rest of the House of Commons but finally concluded that it “passed the wit of man to frame any distinct, thorough-going, universal severance between the one class of subject and the other”. If Gladstone, after 50 years in politics and four terms as PM, could not find an answer, and no government in the next century found it a constitutional possibility, might it not be somewhat immodest for this government to tell us that they had found the answer in just eight weeks? The reality is that having raised the West Lothian question they have not found a workable West Lothian answer, other than by doing what their predecessors refused to do: threatening the very survival of the United Kingdom.

There are Irish lessons we should heed. Ronan Fanning’s book The Fatal Path shows how in our policy towards Ireland from 1910 to 1922 party political interests often trumped the national interest. Keeping Ireland in the United Kingdom was neither the first priority of the Asquith government, which was more interested in tactics to maintain itself in government, nor of the Lloyd George coalition, which could do nothing without appeasing extreme elements on the far right.

David Cameron has taken this one stage further. He has put the integrity of the UK second, not to the real needs of England, but to the very vocal demands of Ukip.

There is a myth that the union can easily survive this new polarisation between Scotland and England because it is held together by longstanding bonds and traditions. But what may have been true in the aftermath of two world wars has given way to a new century where none of our ancient institutions are strong enough or popular enough on their own to bind us together.

The UK will not survive just on the basis of embracing the lowest common denominator – mutual toleration – as we increasingly go our own ways. The United Kingdom will hold together only if there are things that we share together – common interests, mutual needs and similar values – that make us want to cooperate. In the modern world, that has to include a willingness to share risks and transfer resources between each other to ease poverty, unemployment and inequality between the regions and nations.

So I am not advocating the status quo. We should embed in our constitution, not two classes of representation but a Commons reform that allows for detailed debate by English members on English-only measures, but with final decisions on all matters taken by all who are elected. With reform of the Lords, of the regions, of the voting system and of the Commons itself part of the queue of complex, interconnected issues in need of democratic resolution, a constitutional convention that engages Scotland and the rest of the UK is the best way of ensuring we move from an unworkable 19th century constitution to a workable 21st century one.

The former prime minister Lord North is remembered for only one thing – losing America. Will history remember David Cameron for just one thing too – that on the morning of 19 September 2014 he lit the fuse that eventually blew the union apart?