LORD Hailsham famously described the British system of government as an “elective dictatorship”. This was a complaint against the excessive power of the Prime Minister under the majoritarian, winner-takes-all, two-party politics of the post-war era.

As long as a Prime Minister could win general elections and hold together a disciplined majority in the House of Commons, he or she could dominate the policy process. The Prime Minister was head of the cabinet, the chief adviser to the Crown, the leader of the majority party, the repository of all power and patronage, the visible face of the Government, and the one with a democratic mandate to carry out the governing party’s manifesto commitments.

In practice, there were some constraints. One of these was the doctrine of “collective responsibility”. Some Prime Ministers were more forceful, more hands-on, more concerned with the details of government, than others, but they all had to govern through the Cabinet. As Winston Churchill explained to President Roosevelt, “I continuously have to consult and have the support of my Cabinet”.

Even rank-and-file MPs were never entirely subordinate. Labour Prime Ministers had to balance ideological wings of a party that spanned from almost communism to the mildest forms of social democracy. Conservative Prime Ministers generally had a freer hand, but a restive Chair of the 1922 Committee (which represents Tory backbenchers) could create trouble if the “Squires of the Shires” felt things were not going their way.

The civil service was respected for its impartiality and professionalism. The independence of the judiciary was a firm principle. Other institutions – the BBC, the established churches, local authorities, the great universities – were recognised as having autonomous spheres of action, in which they could contribute to the common good according to their distinct purposes, while remaining largely free from government control.

The strongest check was democracy itself: free, fair and regular elections, through which government could ultimately be held responsible. The electoral system was crude, but it was rooted in a robust democratic culture where civil liberties, including freedom of expression, were largely respected. Parliamentary procedure gave space to the Opposition and minor parties. The Government could get its way, but the Opposition could always have its say.

The British political system, as it operated in its mid-20th century heyday, was far from perfect. It sorely needed reform. Yet it was not without its merits. It worked, on the whole, better than tolerably well. The rules that constituted it were, for the most part, understood, broadly accepted as legitimate, and self-enforced. Absolute power was never denied, but it was used rarely and sparingly.

That is no longer the case. In a process that began under Margaret Thatcher and has intensified under Boris Johnson, self-restraint and moral responsibility have been abandoned. The authority of convention has dissolved. The “Good Chaps” theory is no more. The British constitution is now a free-for-all in a game without rules.

This week, the House of Commons abolished Evel (English Votes for English Laws). The passing of this improvised and unworkable solution to the asymmetry of devolution will not be mourned. Nevertheless, the fact that such a change to the legislative process in England could be made in the blink of an eye – without debate, without public consultation – is worrying. They call themselves “Conservatives”, but they level and uproot everything that stands in their path.

It is not an isolated development. The Police, Crime, Sentencing and Courts Bill bulldozes through the right to protest. The Elections Bill will strip many – especially the poorest and most marginal citizens – of their right to vote. Johnson’s constitutional wrecking ball is also taking big swings at human rights, judicial review, and devolution. All this is indicative of the moral disintegration and institutional destruction of what used to be called the “British constitution”.

When public power ceases to operate according to accepted rules and principles it has become despotic. The essence of despotism is not necessarily that it is brutal, but that it is arbitrary, capricious and unpredictable. It does not allow itself to be bound. It recognises no constitutional limits or moral standards. It makes it up as it goes along and calls it “flexible”.

This is anathema in Scotland. Scottish constitutional thought – from the Declaration of Arbroath, through the National Covenant, to the Claim of Right – has consistently rejected absolutism.

The Scottish principle is that sovereignty resides in the whole community of the realm and that all public authorities must in consequence be limited and constitutionally bound. As we stand amid the ruins of the British constitution, Scotland must find the boldness to reassert these principles.

A commitment to a written constitution is central to the pitch for independence. It guarantees that the Scottish state will have sure legal, moral and institutional foundations. That means agreeing upon a draft constitution before the next referendum, so that it is ready to go on Independence Day.

Times Columnist Hugo Rifkind is the guest on the TNT show live on Wednesday at 7pm