Unlike most modern states, Britain does not have a codified constitution but an unwritten one formed of Acts of Parliament, court judgments, and conventions.

For most people, especially abroad, the United Kingdom does not have a constitution at all in the sense most commonly used around the world – a document of fundamental importance setting out the structure of government and its relationship with its citizens. All modern states, saving only the UK, New Zealand, and Israel, have adopted a documentary constitution of this kind, the first and most complete model being that of the United States of America in 1788. However, British people certainly say that they have a constitution. But it exists in an abstract sense, comprising a host of diverse laws, practices, and conventions that have evolved over a long period. The key landmark is the Bill of Rights (1689), which established the supremacy of Parliament over the Crown following the forcible replacement of King James II (r. 1685–88) by William III (r. 1689–1702) and Mary (r. 1689–94) in the Glorious Revolution (1688).

From a comparative perspective, Britain has what is known as an 'unwritten constitution.' Although some prefer to describe it as 'uncodified' on the basis that many of the laws of a constitutional nature are written down in Acts of Parliament or law reports of court judgments. This aspect of the British constitution, its unwritten nature, is its most distinguishing characteristic.