Where does the UK (United Kingdom) Parliament get its power from?

The United Kingdom has an evolve system of common law. The closest thing they have to a constitution is the Magna Carta, which is a doent drawn up in the 1200s on agreement between a group of aristocrats and the King, which outlined their powers in a crude sense. However the UK Parliament today carries the Authority of Queen Elizabeth the second, with her blessing, who, through hereditary means is the British head of state.