The foundation of british government and of constitutional governments around the world is the

That is the Magna Carta.

Magna Carta or the full name Magna Carta libertatum was signed by King John in England on 15 June 1215. Magna Carta which means the Great Charter of the Liberties became the foundation of the British government and the similar constitutional ones around the world as it first introduced and made possible the idea of the limited government. It effectively muzzled royal authority with the constitution and the subsequent laws.