Now certainly there were attempts at compiling and codifying the law, notably associated with the names of King Alfred the Great and Edward the Confessor, but the common view before Maitland was that the law itself was pre-existent. (A recent book that I confess I have yet to read but by the very thorough historian Patrick Wormald is Making of English Law: King Alfred to the 12th century). This was not the exclusive view, some maintained that the Common Law came over from Normandy with the Conquest in 1066, but generally the idea that King Alfred and then Edward the Confessor were law compilers and not law makers won out. (Though Alfred seems to have a more expansive view of his powers in this regard.)
What does any of this have to do with the Magna Carta? And what is so important about the Magna Carta to start with? I mean this is an economics blog! Well I am not sure, I am kind of making it up as I go along, those who want to follow and correct this line of thought can follow along.
A translation of the Magna Carta can be found at Constitution.org http://www.constitution.org/eng/magnacar.htm where it is after the Athenian Constitution held as the second oldest foundational documents for our own Constitution. Yet in reading the Magna Carta it is with a few partial exceptions not foundational at all, instead it is a restoration of the Ancient Law to the time before innovations by King John, his brother Richard the Lion-Hearted and their father Henry II. Now there are two provisions which are foundational if not actually considered original to the Magna Carta.
14. And for obtaining the common counsel of the kingdom anent the assessing of an aid (except in the three cases aforesaid) or of a scutage, we will cause to be summoned the archbishops, bishops, abbots, earls, and greater barons, severally by our letters; and we will moveover cause to be summoned generally, through our sheriffs and bailiffs, and others who hold of us in chief, for a fixed date, namely, after the expiry of at least forty days, and at a fixed place; and in all letters of such summons we will specify the reason of the summons. And when the summons has thus been made, the business shall proceed on the day appointed, according to the counsel of such as are present, although not all who were summoned have come. & 63. Wherefore we will and firmly order that the English Church be free, and that the men in our kingdom have and hold all the aforesaid liberties, rights, and concessions, well and peaceably, freely and quietly, fully and wholly, for themselves and their heirs, of us and our heirs, in all respects and in all places forever, as is aforesaid. An oath, moreover, has been taken, as well on our part as on the art of the barons, that all these conditions aforesaid shall be kept in good faith and without evil intent.Thus Ch. 14 lays down the principle of 'no taxation without representation' while Ch. 63 affirms the pre-existing rights of all Englishmen. But note that these two do not precisely map, you will look in vain for the concept of 'democracy' tout court in the Magna Carta, there is no hint that the Parliament promised in ch. 14 includes FULL representation of the people.
Instead if you read through the Great Charter, the Magna Carta, you will see that it is primarily a reassertion of the King's Tenants in Chief's (those who held their land directly from the King) property rights against the King rights with only some secondary protections for sub-tenants and freemen.
But words and concepts you will not find in the Magna Carta: Equality and Democracy. Instead it is a fairly straightforward defense of the principle that Liberty=Freedom from coercion from above over property, where property includes ones own person. This is not to concede that the concepts of equality and democracy are not inherent in English and hence American society all along, just to recognize that it would be a long time before those were recognized in law.