A written constitution is a formal document defining the nature of the constitutional settlement, the rules that govern the political system and the rights of citizens and governments in a codified form.
The UK's constitution is not written in a single document, but derives from a number of sources that are part written and part unwritten, including accumulated conventions, works of authority, Acts of Parliament, the common law, and EU law.
Historically, the UK has not had a definable statement of individual rights and freedoms either - the 1689 Bill of Rights sets out the powers of parliament vis a vis the monarch - but rather relies on the notion of residual freedom and the concept of parliamentary sovereignty.
Therefore, individuals' rights remain dependent on ad hoc statutory protection or upon judicial protection under common law.
This contrasts to many European and Commonwealth countries and the United States, which have a clearly defined constitutional settlement.
The closest thing the UK has to a bill of rights today is the Human Rights Act 1998, which incorporates the European Convention of Human Rights 1950 (ECHR) into domestic law.