The Supreme Court's decision today is most welcome. Its clarity, its unanimity and its reasoning are exemplary. The British version of democracy that we have enjoyed has been rescued for the moment.

It is a triumph for a constitution that goes back to 1688. Perhaps its finest moment. But, ironically, its finest moment may also be its last. For the very fact that it has had to make this ruling demonstrates that the conventions and political culture that were its foundations have been blasted into the past.

The Brexiteers will strike back ferociously. The Daily Telegraph was already calling for a ‘Brexit Constitution’ to make judges answerable to politicians and bring them to heel, as in today’s United States. Today, in immediate and therefore prepared response to the Supreme Court judgment it has published a diatribe by the Brexit Party MEP John Longworth denouncing the ‘usurper parliament’ and describing the judiciary as part of a continental conspiracy against the spirit and culture of the Anglo-Americans.

As we now know, such raving must be taken seriously. The source is not the casual narcissism of a Prime Minister on a do-or-die mission or the vainglorious intellectualising of his Brexit supremo Dominic Cummings. It is the huge set of interests that oppose all EU regulation and that especially want to protect British tax havens for the international investor class. Johnson and Cummings are the shock troops of their shock doctrine.

So the first thing everyone needs to embrace is that constitutions are not dry and arcane matters for lawyers and academics. Constitutions have complex parts just like the engine of your car or train, that need specialist mechanics. But constitutions are also about what kind of country we are, to whom our system of government belongs and the nature of its democracy. You don’t ask a mechanic to tell you what journey you should take and you don’t need to understand how the engine works to decide where you want to go and care about the vehicle that will take you there.

For three decades I have been told, by Labour officials especially, that the constitution is not of interest to voters ‘on the doorstep’. Then 17.4 million vote to ‘take back control’. Time and again I and others have lobbied Labour’s front bench to pull their fingers out and demand a democratic constitution because, we said, democracy does indeed matter to people. Back I the last century I was rebuffed by Tony Blair. Gordon Brown, Ed Miliband listened at least, but sat on their hands. When Jeremy Corbyn stormed the leadership numerous experienced Labour activists, such as Hilary Wainwright, Jeremy Gilbert, Neal Lawson and the Compass group, not to speak of democracy campaigners, tried to persuade Labour front bencher Jon Trickett (who currently holds the brief for a constitutional convention) to unleash the energy for democratic reform that has been building up. To no avail.

Now we have a new and dangerous confrontation. The Prime Minister claimed the lack of written rules means he can close down parliament when he likes and, in effect, that he and his advisors are beyond the law when it comes to exercising prerogative power. The Supreme Court disagreed. They held their nerve and dismissed such claims outright. But by doing so the judiciary has been forced by the Prime Minister’s own reckless actions to cast aside the de facto separation of judges and courts from the high politics of our country.

Now the judges are being attacked for making a ‘land grab’. See tweets today expressing this from Tim Shipman of the Sunday Times and Iain Martin of Reaction. It isn’t a landgrab at all, it is resistance to aggression.

The aggression will continue. This means there are now three possible directions of travel for the British system of government.

First, the Brexit right wins with its demands for revenge and a US-style codification of winner-takes-all politics. And the UK becomes an extension of Trumpania.

Second, in an attempt to defend the UK from this assault the liberal centre seeks to preserve what we have by codifying the status quo. In other words, to turn the informal convention which govern the way people were expected to behave into formal rules. This is implicit in the call the Speaker of the Commons made at the end of his Bingham Centre lecture when he announced that he had been converted to the need for a written constitution. He called for a Speakers Conference or Royal Commission to advise on what to do next. Peter Hennessy has just told today’s BBC World at One that codification will be essential to protect arrangements that depended on good behaviour.

This is a huge and extraordinarily important shift of opinion. Oddly enough, they are finally catching up with Charter 88. But it is too late for that. Just as democracy is nothing unless it is constitutional, so in the 21st century a constitution is nothing unless it is democratic. This means that if we are to create one, the act of creation must be democratic too.

So, the third option is the creation of a democracy we can all call our own. This will demand a democratic process that draws upon the deliberation of citizens assemblies to start us on the way to a new constitution. This is what Caroline Lucas MP, who I have been working with, has just eloquently argued outside the Supreme Court.

The second and third options are not inherently in opposition to each other, for the very process of codification will release demand for democratic accountability (not to speak of the national question in Scotland, Wales, Northern Ireland and indeed England). We need to bring together all those who want a codified constitution to debate and argue how we achieve this. It is not very difficult to write down a constitution. What is hard is making sure that it belongs to us, the people as a whole.