The people as a collective are the ultimate supremacy. Once accomplished, a constitution cannot be changed without the express will of the people -for to do so would be a violation of contract and would negate the authority of those in breech.
Britain Has A Constitution
Britain has a constitution of considerable standing, the foundations of which were established almost 800 years ago with the signing of Magna Carta in 1215, and reasserted 322 years go with the Declaration of Right and Bill of Rights in 1688.