As constitutional democracy, the United Kingdom is governed by a wide range of institutions, many of which provide checks on each other’s powers. Most of these institutions are of long standing: they include the monarchy, Parliament, (consisting of the House of Commons and the House of loads), the office of Prime Minister, the Cabinet, the judiciary, the police, the civil service, and the institutions of local government.
More recently, devolved administrations have been set up for Scotland, Wales and Northern Ireland. Together, these formal institutions, laws and conversions from the British Constitution. Some people would argue that the roles of other less formal institutions, such as the media and pressure groups, should also be seen as part of the Constitution.
The British Constitution is not written down in any single document, as are the constitutions of many other countries. This is mainly because the United Kingdom has never had a lasting revolution, like America or France, so our most important institutions have been in experience for hundreds of years. Some people believe that an unwritten constitution allows more scope for institutions to adapt to meet changing circumstances and public expectations.
(Source: 2nd Edition, Life in the United Kingdom)