In constitutional theory Westminster is supreme and Parliament cannot bind its successors. This is overstated and subject to political checks (and particular issues in the context of Scots law given arguments about “fundamental law” in the Acts of Union) but is sufficient for basic purposes.
Now, if Parliament passes a provision saying that, for example, a two thirds majority is required to vary any royal charter made after a certain date what is the effect?
It means that any future variation of that royal charter will require a 2/3 majority in Parliament.
But, if the provision for the 2/3 majority is in a normal piece of legislation that legislation itself can be amended or repealed by later legislation. Later legislation is passed by a simple majority. So the 2/3 majority rule would simply be a normal statutory rule which could be changed or revoked by 50% plus 1. Unless, of course the legislation containing the provision is itself protected by a rule that provides that that provision (or the broader legislation) cannot be amended by a special majority.
To demonstrate: section X of statute A says “royal charters can only be amended by a 2/3 majority of parliament”. What is to stop an amendment or repeal of section X, or statute A, or both?
Even if the section is itself protected by an entrenching rule that it cannot be amended unless by a 2 /3 majority how is that entrenching rule protected? Potentially by a section or legislation that itself is potentially subject to the normal simple majority rule.
So, good luck with the whole entrenching the 2/3 majority vote thing.
Luckily, most political journalists don’t understand much in the way of constitutional law so they’ll not notice.
ETA – a couple of very helpful comments via twitter
@loveandgarbage And, since the courts will not review the enacting of primary legislation, all moot anyway.
— Matthew Taylor (@MTPT) March 18, 2013
@loveandgarbage see also the discussion in Jackson v. A-G about poss of two-part process to extend Parliament beyond 5 years under PA1911.
— Paul Scott (@PaulFScott) March 18, 2013