The magna carta was a compact between the English monarch and his nobles. It never was really a "constitution" the British constitution is now as it was then an unwritten one. If you look at Blackstone's Commentary on English Laws, many of the things that the Constitution enshrines were commonly understood at the time to be the rights of Englishmen, ie Blackstone on the right to bear arms:
“Self defense is justly called the primary law of nature, so it is not, neither can it be in fact, taken away by the laws of society.”
“In these several articles consist the rights, or, as they are frequently termed, the liberties of Englishmen. . . . So long as these remain in*violate, the subject is perfectly free; for every species of compulsive tyranny and oppression must act in opposition to one or other of these rights, having no other object upon which it can possibly be employed. . . . And, lastly, to vindicate these rights, when actually violated or attacked, the subjects of England are entitled, in the first place, to the regular administration and free course of justice in the courts of law; next, to the right of petitioning the king and parliament for redress of grievances; and, lastly, to the right of having and using arms for self-
preservation and defense.”
