While the United Kingdom does not have a codified constitution that is a single document, the collection of legal instruments that have developed into a body of law known as constitutional law has existed for hundreds of years.
As part of this uncodified British constitution, constitutional conventions play a key role. They are rules that are observed by the various constituted parts though they are not written in any document having legal authority; there are often underlying enforcing principles that are themselves uncodified. Nonetheless it is very unlikely that there would be a departure of such conventions without good reason, even if an underlying enforcing principle has been overtaken by history, as these conventions also acquire the force of custom.
Conventions are often thought of as "unwritten" but sometimes conventions are recorded in writing e.g. the Cabinet Manual, the Ministerial Code. Similarly conventions are often thought of as evolving over time but they can sometimes be created at a specific moment in time e.g. the Salisbury Convention.
Examples include:
Treaties, although ratified using Royal Prerogative, will not be ratified until the passing of a suitable statute law by Parliament. This is necessary if the treaty requires an amendment to domestic law, affects the rights of private individuals, requires public expenditure, grants the Crown additional powers, or cedes territory. Examples include extradition treaties, double taxation treaties, and reciprocal social-security treaties.
The monarch will accept and act on the advice of their ministers, who are responsible to Parliament for that advice; the monarch does not ignore that advice, except when exercising Reserve powers.
The Prime Minister of the United Kingdom is the leader of the party (or coalition of parties) with an absolute majority of seats in the House of Commons and therefore most likely to command the support of the House of Commons.
All money bills must originate in the House of Commons.
The monarch will grant a dissolution of Parliament if requested (1832 to 2011 – the Lascelles Principles in 1951 informally outlined the principles and issues that might lead to a refusal of a dissolution).
The Prime Minister alone advises the monarch on a dissolution of Parliament (1918 to 2011).
These conventions were implicitly ended by the Fixed-term Parliaments Act 2011, which removed dissolution of Parliament from the Royal Prerogative altogether.
The monarch grants the Royal Assent to all legislation – sometimes characterised as all legislation passed in good faith. It is possible that ministers could advise against giving consent, as happens with the Crown dependencies (convention since the early 18th century – previously monarchs did refuse or withhold the Royal Assent).
The Prime Minister should be a member of either House of Parliament (between the 18th century to 1963).
In 1963 it was amended to the effect that no Prime Minister should come from the House of Lords. When the last Prime Minister peer, the Earl of Home, took office he renounced his peerage, and as Sir Alec Douglas-Home became an MP.
The Prime Minister can hold office temporarily whilst not a Member of Parliament, for example during a General Election or in the case of Douglas-Home, between resigning from the Lords and being elected to the Commons in a by-election.
All Cabinet members must be members of the Privy Council, since the cabinet is a committee of the council. Further, certain senior Loyal Opposition shadow cabinet members are also made Privy Counsellors, so that sensitive information may be shared with them "on Privy Council terms".
The House of Lords should not reject a budget passed by the House of Commons. This was broken controversially in 1909 by the House of Lords, which argued that the Convention was linked to another Convention that the Commons would not introduce a Bill that "attacked" peers and their wealth. The Lords claimed that the Commons broke this Convention in Chancellor of the Exchequer David Lloyd George's budget, justifying the Lords' rejection of the budget. The Commons disputed the existence of a linked convention. As a consequence, the Lords' powers over budgets were greatly lessened by the Parliament Act 1911.
During a General Election, no major party shall put up an opponent against a Speaker seeking re-election. This convention was not respected during the 1987 General Election, when both the Labour Party and the Social Democratic Party fielded candidates against the Conservative Speaker, Bernard Weatherill, who was MP for Croydon North East. The Scottish National Party (SNP) does stand against the Speaker if he or she represents a Scottish Constituency, as was the case with Michael Martin, Speaker from 2000 to 2009.
The Westminster Parliament will only legislate on reserved matters. It will not legislate on non-reserved matters ("devolved matters") without first seeking the consent of the Scottish Parliament (since 1999, the Sewel convention, later renamed to Legislative Consent Motions).
The House of Lords shall not oppose legislation from the House of Commons that was a part of the government's manifesto (the Salisbury Convention).
