The document serving as the foundation for the British constitution, delineating civil rights, is the Magna Carta, signed in 1215. This historic charter was a crucial step in limiting the powers of the monarchy and ensuring certain legal rights for subjects. It introduced principles such as the rule of law, due process, and the concept of parliamentary oversight. Over time, the Magna Carta evolved into a symbol of liberty and justice, influencing constitutional development across the globe and laying the foundation for modern democratic governance and civil rights protection.