I am not sure to what extent Magna carta was influenced by Anglo Saxon law, custom and civil service. the norman conquest pretty much destroyed the saxon ruling class and levers of government. The new ruling class appear to have retained much of the lower civil service and county structures which appear to have worked well from what i have read. Maybe the new rulers were influenced by these saxons and adopted the saxon view of monarchy.... if a single saxon view existed. A lot of English legal tradition was absorbed from the Northern Danish/Norse counties. The England that the normans took still had clear divides and may have been part of the reason for the Norman eradication of the northern english to remove the possibility of a power base for norse or dublin vikings.

The maga carta was imposed by norman decendants on a norman kings decendant ... to what extent they took into account english custom is dificult to estimate.