On the surface, at least, parliamentary sovereignty — a phenomenon that applies to the UK, or Westminster, Parliament, but not to the UK’s devolved legislatures — is a simple concept. To paraphrase Dicey, Parliament has the legal authority to enact, amend or repeal any law, and no-one has the legal authority to stop it from doing so. But this notion is as extravagant as it is simple: it means, as Stephen famously put it, that a law directing the killing of all blue-eyed babies would be valid. The fact that such laws remain unenacted is thanks to “political constitutionalism” as opposed to “legal constitutionalism”: it is political, not legal, factors — including, one hopes, legislators’ own sense of morality — that operate as the restraining force.
It is often assumed that the sovereignty of Parliament follows from the absence in the United Kingdom of a written constitution, the existence of such constitutions generally being associated with legislatures that have only limited powers. Within such legal systems, the written constitution usually performs two functions (which are in truth flip sides of one coin) that rule out anything like parliamentary sovereignty. The constitution confers authority on the legislature; and the constitution restricts the legislature’s authority (by omitting to confer the power to do certain things). Within this type of constitutional framework, the legislature only has those (limited) powers that the constitution grants: and if the legislature attempts to make laws beyond the powers granted to it, then (often) courts can intervene by quashing unconstitutional legislation (or simply refusing to apply it, on the ground that it is not really “law”).