Bramshott
You have a point. Parliament (at Westminster) can legislate in respect of matters that the GLA is empowered to deal with. However, the same is true of matters devolved to the Scottish Parliament. TheChandler is correct in that there is a constitutional convention that Parliament won't do this without consent, but Parliament is entitled to ignore this (conventions aren't enforceable - they are like gentlemen's agreements).
However, there's not much comparison between the GLA and the SP. For starters, the GLA has far, far less powers than the SP. Secondly, Scotland is a large geographical proportion of the UK and has always had in some sense a separate administration, and it has always had a separate legal jurisdiction (as does NI). There's no practical reason for excluding Parliamentary intervention in London's affairs
OTheHugeManatee
The arrangement looks a mess, but if it works, who cares?
A lot of concern arises from the fact that Britain's constitution looks muddled and messy because it's "unwritten". All this actually means is there is no single document called "the constitution". This is a very good thing because it means lawyers can't get very rich arguing about what a bunch of men who died 200 years ago actually meant, or whether their views prevent a democratically elected goverment from doing things the majority want now (two problem our friends in the US suffer from).
The British constitution is actually very simple. It is basically that there is one Parliament, which can pass whatever law it likes. It is made of three parts: the House of Commons (the boss), the House of Lords, and the monarch, all of which must agree to any laws made by Parliament. Also there is an independent judiciary that interprets what those laws mean, applying case law principles as they do so.
It also provides that the monarch governs through her ministers. (in practice, this is under laws passed by Parliament allowing it do do what it needs to do).
That's basically it. There is no high falutin' document drafted on the basis of grand principle, and a good thing too. Law is a tool for resolving disputes, not a vehicle for philosophical statement.
Within this very flexible system, it's easy to carve out some powers and hand them over to a particular body (such as the Scottish parliament) and allow them to legislate essentially on Westminster's behalf. And it's also perfectly easy for Parliament to regulate its own procedure by barring some of its MPs on voting on certain issues, and for it to remove the bar. Both are done by a simple vote.
By contrast, creating an English parliament (Bwaaah! Bwaah! Scotland's got one - I want one too! It's not fair!!) would either create the daft situation whereby the supreme lawmaking authority in the UK basically had nothing to do, or was permanently gutted of its authority, thus ripping up centuries of legally established common-sense and practice. That would be a seriously daft thing to do.
NB: there are two other countries in the world that have "unwritten" constitutions: Israel and New Zealand. New Zealand's constitution is basically identical to the UK's. Like the UK it is a comparatively well-run country by international standards, so it seems to work there too.