This is not strictly true, however. There are written documents that establish the constitutional make-up of the State, and the earliest is Magna Carta. They also include:

• The Habeas Corpus Act of 1679, which enshrines the principle that nobody may be arbitrarily detained without having their case heard in a court of law.
  
  
• The Bill of Rights of 1689, which followed the deposing of James II.
  
  
• The European human rights provisions that were incorporated into UK law in 1998.

Other than that, though, it is true to say that most of the way the State actually works, in terms of the making and enforcing of laws, is subject to centuries-old conventions that have never been formally codified – or, as constitutionalists put it, entrenched – in the form of written guarantees.

The main features are:

Statute law

Laws are proposed and enacted by the two houses of Parliament, the Commons and the Lords. A bill is put before Parliament, debated and voted on, goes through committee stages, where its workings and its potential effects are scrutinised, and once it receives its third and final reading, it enters the statute books and becomes the law of the land. The accumulated body of all such laws enacted in this way is known as statute law.

Common law

Also known as case law, common law is made up of the legal principles that emerge from individual court cases. Not every apparent infringement of the law is clear-cut, and it is up to a court of law to hear the evidence on all sides before arriving at a verdict based on the interpretation of existing law. Certain rulings by judges go on to create legal precedents, which are then cited in future cases, so that a particular test of the law helps to shape it as much as what is actually written down in the statute books.

The supremacy of Parliament

All executive power resides ultimately with Parliament. No other body is capable in practice of overruling the decisions of Parliament. Laws flow from Parliament, and the House of Lords, as well as being a legislative chamber, is also the highest court in the land. If your case has failed in a normal court of law, and then in the Appeal Court, you have the right finally to have it heard in the upper chamber of Parliament. Parliament has the right to summon anybody to appear before its committees, and acting in contempt of Parliament is a serious offence.

Division of powers between the executive and the monarch

Although the monarch retains certain constitutional powers, such as the ability to declare war on another territory, to dissolve Parliament and to appoint archbishops, these powers are never exercised in practice. Even appointments to the clergy are made by the Prime Minister, who seeks the monarch’s consent to a decision effectively made at governmental level. These arrangements flow from the turbulent 17th century, when England became a Commonwealth, before a monarchy with severely curtailed political power was restored in 1660.

Actual executive power flows from whichever party has a majority in the House of Commons. As each law passes on to the statute, it must receive royal assent, which means that the monarch must agree to it and sign it. The last time a bill failed to receive royal assent was in 1708, when Queen Anne declined to assent to the Scottish Militia Act.

Independence of the judiciary

The workings of the courts are theoretically free from all political interference. Politicians cannot intervene to direct the verdicts of juries or the rulings of judges, which are bound only by the body of preceding statute and common law. In recent years, there have been attempts by governments of both the principal political parties to exercise influence over such matters as sentencing policy, so this aspect of the constitution may currently be seen as being in a state of flux.

Conventions that do not have the force of law

There is a strong element in British constitutional affairs of things happening just because this is the way they have always been done, or at least have been for a very long time. Tradition dictates that the State Opening of Parliament must begin with Black Rod, the monarch’s messenger, having the door to the Commons chamber slammed in his face when he comes to summon members to the House of Lords. This symbolises the constitutional supremacy of the elected house, which is able to defy the wishes of peers.

The conventions also cover the code of conduct for ministers of the Crown, who are expected to conform to certain standards of behaviour. A secretary of state who has lost the confidence of the House, particularly of those on his or her own side, is expected to do the decent thing and resign. Similarly, a minister who is found to have told a deliberate untruth has to go. The conventions are not legally enforceable, but have been almost invariably observed throughout history.

Authoritative works

From time to time, writers on Britain’s constitutional affairs have sought to summarise and codify the arrangements by which the nation is governed. The more authoritative of these works themselves then acquire the status of constitutional documents. The 19th century was the great era of such writers, producing Walter Bagehot’s The English Constitution (1867), AV Dicey’s An Introduction To The Study Of The Law Of The Constitution (1885) and Thomas Erskine May’s A Practical Treatise On The Law, Privileges, Proceedings And Usage Of Parliament (1844). The last became the bible of parliamentary procedure, has been regularly updated ever since its first appearance, and is always consulted in the event of a procedural dispute.

International treaties and agreements

The UK is also bound, like all countries, by whatever international treaties and accords it has signed up to. These include various treaties of the United Nations and the Treaty of Rome, by which we entered what was then known as the European Economic Community. Rulings of the European Union now theoretically have precedence over the national Parliament, although so far there has been no major constitutional clash between  the two. The so far troubled attempt by the EU to produce its own written constitution, which all member states will be invited to ratify, may well result in a shift in the nation’s constitutional centre of gravity in years to come.

The media?

Some would argue that the investigative role of the news media has, in recent times, begun to exert a decisive influence over the way the country is run. Revelations about the private indiscretions of ministers, together with the pursuit of a sustained campaign to unseat them from office, often succeeds in ending a career. Although the executive may resent this as representing an encroachment on the supremacy of Parliament, governments nonetheless increasingly court the support of media proprietors when a general election is in the offing. For their part, news journalists argue that they are only performing the time-honoured role – guaranteed by the freedom of the press – of questioning the actions of the executive authorities.

Resistance to change

While aspects of the way constitutional authority is applied have grown increasingly contentious in the era since 9/11, there is still a sense that any unwarranted encroachment on its spirit will be stiffly resisted by the people, the more so since there is comparatively little letter to encroach on.

A 19th century American judge, Justice Bradley, put the point well in the course of delivering his opinion in a case heard in Louisiana in 1873:

“England has no written constitution, it is true; but it has an unwritten one, resting in the acknowledged, and frequently declared, privileges of Parliament and the people, to violate which in any material respect would produce a revolution in an hour.”