Constitutions organise, distribute and regulate state power. They set out the structure of the state, the major state institutions, and the principles governing their relations with each other and with the state's citizens. Britain is unusual in that it has an 'unwritten' constitution: unlike the great majority of countries there is no single legal document which sets out in one place the fundamental laws outlining how the state works. Britain's lack of a 'written' constitution can be explained by its history.
In other countries, many of whom have experienced revolution or regime change, it has been necessary to start from scratch or begin from first principles, constructing new state institutions and defining in detail their relations with each other and their citizens. By contrast, the British Constitution has evolved over a long period of time, reflecting the relative stability of the British polity. It has never been thought necessary to consolidate the basic building blocks of this order in Britain. What Britain has instead is an accumulation of various statutes, conventions, judicial decisions and treaties which collectively can be referred to as the British Constitution. It is thus more accurate to refer to Britain's constitution as an 'uncodified' constitution, rather than an 'unwritten' one.
It has been suggested that the British Constitution can be summed up in eight words: What the Queen in Parliament enacts is law. This means that Parliament, using the power of the Crown, enacts law which no other body can challenge. Parliamentary sovereignty is commonly regarded as the defining principle of the British Constitution. This is the ultimate lawmaking power vested in a democratically elected Parliament to create or abolish any law. Other core principles of the British Constitution are often thought to include the rule of law, the separation of government into executive, legislative, and judicial branches, and the existence of a unitary state, meaning ultimate power is held by 'the centre' - the sovereign Westminster Parliament. However, some of these principles are mythical (the British constitution may be better understood as involving the fusion of executive and legislature) or in doubt (Parliamentary sovereignty may now be called in question given the combined impact of Europe, devolution, the Courts, and human rights).
The British Constitution is derived from a number of sources. Statutes are laws passed by Parliament and are generally the highest form of law. Conventions are unwritten practices which have developed over time and regulate the business of governing. Common law is law developed by the courts and judges through cases. The UK's accession to the European Communities Act 1972 has meant that European law is increasingly impacting on the British Constitution. The UK is also subject to international law. Finally, because the British Constitution cannot be found in any single document, politicians and lawyers have relied on constitutional authorities to locate and understand the constitution.
An uncodified constitution creates two problems. First, it makes it difficult to know what the state of the constitution actually is. Second, it suggests that it is easier to make changes to the UK Constitution than in countries with written constitutions, because the latter have documents with a 'higher law' status against which ordinary statute law and government action can be tested, and are only amendable via elaborate procedures. The flexibility of the UK constitution is evident from the large number of constitutional reforms since 1997, including the abolition of the majority of hereditary peers in the House of Lords, the introduction of codified rights of individuals for the the first time in the Human Rights Act 1998, and devolution to Scotland, Wales and Northern Ireland. Arguably, however, these recent constitutional reforms may have made the constitution less flexible in some respects: it is debatable, for instance, whether the devolution settlements could ever be repealed.
For in depth notes on a range of constitutional issues see the House of Commons Library.