A constitution is said to be un-codified when there is no one document or even series of formally related documents that can be identified as "The Constitution". Britain, like Israel is said to have an un-codified constitution. No constitution however is entirely un-codified in the sense that none of its major rules are written in formal documents. Some of the major rules of the British Constitution are written in statute law, which is made by Parliament. Other major constitutional rules are contained in the common law, which is judge-made law that has developed over the centuries.
Statute law, which is made by Parliament, is one main source of the UK Constitution. For example, the Parliament acts of 1911 and 1949, which regulate the maximum duration of Parliament and the powers of the House of Lords, are clearly part of the "law of the constitution".
Other constitutional rules are contained in the common law (judge made law that has developed over the centuries); this is also a source of the UK Constitution. For example, the royal prerogative, that is, the powers legally left in the hands of the Crown, is a product of the Common law. The Prime Minister is appointed and Parliament is dissolved by royal prerogative.
Britain ratified the European Convention on Human Rights in 1951, and since 1955 has accepted the right of individual petition to the European Commission on Human Rights, thus making EU law a source of the UK Constitution.
The sources of the UK Constitution I have given thus far have been constitutional rules, which are enshrined in law; some major constitutional rules/sources of the UK Constitution are contained in the conventions of the Constitution, that is rules that are not law. For example, it is a convention that the monarch must assent to any bill passed by the two Houses of Parliament. Even some conventions, however, are codified in the sense that they are contained in formal documents.