https://www.ucl.ac.uk/constitution-unit/explainers/what-uk-constitution
The UK is often said to have an 'unwritten' constitution, but this is not strictly correct; it is largely written, but in different documents. It has never been codified; brought together in a single document. In this respect, the UK is different from most other countries, which have codified constitutions. But not all: New Zealand and Israel also lack a codified constitution.
Codified constitutions are typically produced following a major historic turning point, such as the grant of independence, revolution, defeat in war, or complete collapse of the previous system of government. None of these things have happened to the UK, which is why it has never had cause to codify its constitution. (Our one revolution, in the 17th century, did briefly produce a codified constitution: Cromwell’s Instrument of Government).
This is the reason why the UK has not felt the need to codify its constitution. But the UK does have a constitution, to be found in leading statutes, conventions, judicial decisions, and treaties. Examples of constitutional statutes include the Bill of Rights 1689, Acts of Union 1707 and 1800, Act of Settlement 1701, Parliament Acts 1911 and 1949, Human Rights Act 1998, Scotland Act, Northern Ireland Act and Government of Wales Act 1998. Examples of conventions include that the monarch acts on ministerial advice; that the Prime Minister sits in the House of Commons; that the Queen appoints as Prime Minister the person most likely to command the confidence of the House of Commons. These and other conventions have themselves been codified in documents such as the Cabinet Manual.
Parliamentary sovereignty is commonly regarded as the defining principle of the British Constitution. This is the ultimate law-making power vested in the UK parliament to create or abolish any law. But parliament can limit its law-making power, as in the Human Rights Act; or devolve legislative power, as in the Scotland Act. Other core principles of the British Constitution include the rule of law, the separation of government into executive, legislative, and judicial branches, the accountability of ministers to parliament, and the independence of the judiciary.
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