All I can tell you is that Magna Carta is often invoked as a founding document of the rights of the common people over the whims of the monarch... It placed the word of law above the word of the king.
Although, when I say common people, obviously I mean landowning aristocrats. Actual common people barely even counted as people in those days... An attitude that still seems to be politically expedient from time to time.
Rather a sad figure, King John, as kings go. He had a hard enough time of it being the bad guy who had to raise taxes to fund King Richard's crusades while Richard, the guy spending the money, got all the glory for being out there doing the fighting when really he should have been at home running his own country.
Once John became King in his own right we see him being browbeaten into signing away a good deal of his allegedly God-given power.
Interesting that there's never been a King John II. Whenever we have a monarch we're a bit embarrassed about we seem to stop using the name...hence, no Richard IV, William V, or Charles III. Not yet anyway.