A CONSTITUTION is the fundamental or basic law which establishes the framework of government. Typically, a constitution codifies the major duties, powers and functions of the institutions of government and the rights and duties of individuals. Britain’s constitution is very different from those of other western nations, reflecting its history.
Britain’s constitution has evolved over hundreds of years, and unlike, say France, Germany, Russia and the USA no revolutions or enemy invasions have wiped away our constitutional arrangements and caused us to start afresh.
Britain’s constitution is uncodified (unwritten). This is not to say that it doesn't exist: rather that there is no single document which explains the framework of government. To find out about the British constitution you would have to refer to a mixture of Statute Law (Acts of Parliament), Common law (derived from precedents and customs), Conventions (long-established procedures), Works of Authority (text books by constitutional scholars like Bagehot and Dicey, and now European Union law. The USA and all other European countries have codified (written) constitutions, which have higher status than ordinary law.
This means that Britain’s constitution is flexible, with no special legal procedure for changing (amending) the constitution, meaning for instance Parliament could change the composition of the House of Lords by passing the 1999 Act, with the Judiciary having no power to challenge that. This means though that the way the UK is governed can be adjusted to match the needs of the current society.
Other countries, with codified constitutions, have to follow special procedures to amend their constitution, making them quite inflexible. So Germany’s first 20 articles can never be amended, Denmark and Ireland require referendums, and the USA needs a 2/3rds majority in both Houses of Congress and the approval of the 50 state legislatures. In the latter case this explains why the US constitution has had only 27 amendments since 1787.
Britain’s constitution is also unitary. The UK has no states, so whilst Central government can choose to give out powers to devolved assemblies and local councils it can take those powers back at any time as they are not guaranteed by the constitution. This has happened with Northern Ireland when, for instance, the Stormont government was suspended as the Nationalist and Unionist politicians couldn’t agree on power sharing. This is contrasted with federal systems like the USA, Germany, power is divided between a central (federal) government and various states (shared sovereignty).