In addition to legislation and judicial decisions, there are other legal sources of the constitution. These include the Royal Prerogative and Parliamentary law and customs.
The power of Government is derived from legislation passed by Parliament. Ministers will often be authorised to take decisions on a particular matter by an Act of Parliament, so are granted statutory powers.
There are also prerogative powers. Under Dicey’s definition the royal prerogative is
.. the residue of discretionary or arbitrary authority which, at any given time is legally left in the hands of the Crown... Every act which the government can lawfully do without the authority of an Act of Parliament is done in virtue of this prerogative... (An Introduction to the Study of the Law of the Constitution (1885)).
Historically, these powers were part of the common law as a bridge between times of absolute monarchy to parliamentary supremacy. In modern times, some powers remain as part of tradition but are effectively exercised by the Government not the monarch. There are some key royal prerogative powers that still exist.
Domestically these include the summoning of Parliament, appointment and dismissal of the Prime Minister, Royal Assent to Bills, deployment of the armed forces within the UK (defence of the realm) and granting of honours. In relation to foreign affairs powers include declaration of war and deployment of troops overseas, concluding of international treaties and recognition of foreign states.
Parliamentary custom is broad and uncertain. It can be seen as being based on the common law
privilege of each institution to regulate its own proceedings and includes authority to regulate MPs behaviour and expel members. Privileges also relate to freedom of speech in Parliament and legal immunities.
The concept can be found in Article 9 of the Bill of Rights 1689, which prohibits interference with Parliament by outside bodies.
Bill of Rights 1689
.. freedom of speech and debates or proceedings in Parliament ought not to be impeached or questioned in any court or place out of Parliament....
In 1999, the Joint Committee on Parliamentary Privilege set out the modern interpretation of Article 9
.. The modern interpretation is now well established: that article 9 and the constitutional principle it encapsulates protect members of both Houses from being subjected to any penalty, civil or criminal, in any court or tribunal for what they have said in the course of proceedings in Parliament....
Authority for Parliamentary law and custom is found in Erskine May's Parliamentary Practice, which is edited by the Clerk of the House of Commons. This reference book is a secondary not primary source of law. There are four primary sources, practice, Standing Orders, Speaker's rulings and statute.
Precedent is crucial in the functioning of Parliament and means established procedures will be followed unless explicitly altered in statute. Processes can be found through evidence from the official
Journals of the House. For example, the rule that each Bill is read three times in each House is followed as a matter of precedent.
Standing Orders are written rules relating to the conduct of Parliamentary business. Standing orders regulate the way MPs behave, Bills are processed and debates are organised. An order is passed by a majority in a single vote in the House of Commons.
Some are temporary and last until the end of a session in which they are passed. There are around 150 relating to parliamentary business and public Bills and about 250 relating to private business.
The Speaker has been elected by the House to provide guidance and enforce the procedures. The rulings made by the Speaker of the House of Commons can be a precedent for the future.
Statute can relate to Parliamentary procedures. The Statutory Instruments Act 1946 sets out the process for delegated legislation. Such Acts should be enforced by Parliament itself rather than the courts.
The question of enforcing Parliamentary law and custom is complex and raises issues both inside and outside of Parliament.
House of Commons
The Speaker who is primarily responsible for enforcing the law and custom of Parliament. The role includes directing an MP to withdraw remarks if they are inappropriate,
namingwhich means putting forward MPs for suspension and suspending the sitting of the House due to serious disorder
The Sergeant at Arms directs the police on duty in the House and who acts on the authority of the House outside the House.
House of Lords
There is a less formal enforcement procedure. The Lord Speaker has no powers to enforce order and Peers are expected to observe the rules.
Parliamentary law and custom is distinguished from other sources of the UK constitution as it is not enforced by the courts. Statute prevails over Parliamentary custom however, the relationship between the common law and Parliamentary custom lacks clarity. Courts can recognise the existence but not enforce, as that is a matter for Parliament.
Bowles v. Bank of England 
It was found that resolutions have no legal force outside Parliament itself, with the exception of budgetary resolutions.
Stockdale v. Hansard (1839)
It was held that each House was the sole judge of its own privileges and a resolution of the House declaratory of its own privileges could not be questioned in any court of law.
Some works by leading academic commentators can be taken into account by the courts when determining constitutional points. They are not binding authority but can be persuasive. . For example Dicey's Introduction to the Study of the Law of the Constitution or Erskine May's Parliamentary Practice.