Actually, there are many cultures that have romanticised egalitarian social structures, including most western liberal democracies. I am in favour of such social structures, but not in favour of romanticising them! The Arthurian myths are not so easily 'lumped together' though - there is a marked difference between the Mabinogion and the French geste.
While I am a great admirer of William Marshal, and his military abilities and atypical behaviour, I think that you over-estimate the success and effect of the Magna Carta. Representative government in the UK didn't truly exist until the Representation of the People Act in 1928, with female suffrage. Until then it was the domain of select and unrepresentative groupings.