Amidst all the debate surrounding the Magna Carta as its eight hundredth anniversary arrives, one of the most arresting points must surely be the simple fact that elements of it are still the law of the land today.

To quote Friday’s Guardian editorial: ““To none will we sell, to none will we deny, or delay, the right of justice” – chapter 40 of the charter and still in force today – is as resonant and succinct a statement today as it was eight centuries ago.”

It’s a reminder of the United Kingdom’s unique inheritance: a flexible constitution that is comprised of more than 800 years of treaties, laws and legal judgements, in which the best of the past may be preserved – as with the above-quoted portion of the Magna Carta – but without making the present a captive of past decisions.

Yet to some, Magna Carta appears to symbolise the very opposite, acting as a stand-in for a codified constitution on the American model (and the Americans are very fond of it, having co-opted it into their own foundation myth).

Indeed, much of the debate on the suggestion that we need a ‘new Magna Carta’ is expressly about furnishing Britain with just such a supreme document.

Both left and right are playing this game: David Cameron attacks the Human Rights Act as having ‘devalued’ the Magna Carta, whilst left-wingers envision a charter that would restrain those bits of the state they dislike.

What both of these arguments – Cameron’s in particular – neglect is that our rights and freedoms don’t stem from the Magna Carta, but from centuries of Parliamentary law. The ‘Great Charter’ was a step on that long journey, but only a step.

The fallacy of using it as an argument for a codified constitution is illustrated neatly by the fact that most of the Magna Carta isn’t on the statute book anymore.

It was drawn up by competing power blocs in the context of their own time, rather than a panel of philosopher princes, and later legislators overturned great sections of it as time, opinion and power shifted and moved on.

The same shortcomings will be present in any codified constitution imposed on Britain today. The clashes between the various blocs of power and opinion will not be so stark – nor as well-armed – as their Thirteenth Century predecessors, but the divisions will be real, and so will the compromises.

Unalloyed statements of high principle are, unless they are too broad to be meaningful, impossible to enshrine as incontestable in healthy, pluralist democracies.

Furthermore, just as the iron certainties of that baronial age quickly became arcane, so too with the pious philosophies of the architects of our own constitutional grand design fall into discord and disrepute. All things pass.

The real value of Magna Carta lies in its role in the gradual evolution of our unique Parliamentary democracy, and a system wherein the highest constitutional authority in the realm is not a panel of unelected justices, or a demi-pantheon of mythologised ‘founding fathers’, but the elected representatives of the people.

It is that organic, untidy, adaptive quality that has permitted us to keep the best of the Magna Carta whilst jettisoning the rest, and has ensured that the centre of our politics remains Parliament and elections, rather than courts and those special interest groups who can fund litigation, as in the United States.

It is especially ironic that, in venerating this document as a unique national artefact, so many of its supporters seek to strip our constitution of its distinct strengths, substituting instead the tired orthodoxies of American democracy.

It has been remarked elsewhere that the real value – or at least, the ongoing power – of the Magna Carta lies in its myth, rather than fact. But whilst myths have some value, we should not allow ourselves to be taken in by them.

The aristocratic compromise the anniversary of which we mark today was not an early constitution, and does not support the case for trying to bind the hands of later generations by enshrining as supreme the assumptions of our own time.

Just as we would not wish to live under the barons’ laws, we must accept that our children will want to live under their own.