The UK Constitutional Law Association Blog has published a strange take on the European Union (Withdrawal) (No. 2) Act 2019 (aka, the Benn Act) by Michael Detmold wherein he claims that the letter contained in the Schedule to the Act is unconstitutional.
The crux of Detmold’s argument is that this portion of the Benn Act is incompatible with the convention of responsible government because it effectively declares a lack of confidence in the Government while still allowing the Government to remain in office. According to Detmold, “[s]uch a law cannot exist in a regime of confidence, and it should be open to the Supreme Court to hold it unconstitutional on the ground that the law is so alien to fundamental principle that it cannot be recognised as law.”
There’s one big problem with this argument and that’s the doctrine of parliamentary sovereignty. It’s a central tenet of the British constitution that the Queen-in-Parliament has unfettered legislative competence. Now, Parliament may choose to limit that competence. For example, the European Communities Act 1972 gave EU law primacy over UK law, but these limits remain self-imposed and can be amended or repealed at any time.
It follows, therefore, that no court could strike down an Act of Parliament because it is “alien to fundamental principle.” As Lord Reid noted in Madzimbamuto v Lardner-Burke  UKPC 18:
It is often said that it would be unconstitutional for the United Kingdom Parliament to do certain things, meaning that the moral, political and other reasons against doing them are so strong that most people would regard it as highly improper if Parliament did these things. But that does not mean that it is beyond the power of Parliament to do such things. If Parliament chose to do any of them the Courts could not hold the Act of Parliament invalid. 
Consequently, Parliament is free to alter the UK’s constitutional arrangements at any time and to any degree. In other words, Parliament could transform the UK into a presidential republic tomorrow if it wanted to. So even if the Benn Act conflicts with conventional notions of responsible government, that doesn’t make it unconstitutional.
Detmold also suggests that requiring Boris Johnson to request an extension under Article 50 would be akin to Parliament enacting a statute that required a private individual to write a love letter to a total stranger. This analogy is flawed for two reasons. First, an enactment requiring someone to write a love letter to a stranger is in no way comparable to an enactment that requires a public officeholder to exercise a power pertaining to that office. Second, even if Parliament were to pass a law that obliged a private individual to write a love letter to someone else, it would still be perfectly lawful. Silly, yes, but lawful nonetheless. Similarly, Detmold’s later implication that Parliament could not pass a law which declared two strangers to be married is also false. The individuals in question might not consider themselves to be married of course, but that would have no bearing on their legal status (the fact that “their wills have not gone with it” is immaterial).
On a side note, the idea of Parliament legislating on the matrimonial affairs of private individuals is not as absurd as it might seem at first. Until the advent of judicial divorce in the 19th century, the only way to dissolve a marriage was by Act of Parliament. Parliament has also legislated to enable specific couples to marry within the prohibited degrees of relationship!
Detmold’s critique of the Benn Act is without foundation. While the propriety of Parliament’s actions is open to question, their validity is not.
 See also the decision of the House of Lords in R (Jackson) v Attorney General  UKHL 56.
 It should also be noted that statute law takes precedence over the common law.
 Detmold does attempt to reconcile his argument with the concept of parliamentary sovereignty, but it doesn’t work. He claims that “the Dicey rule [regarding the limits of parliamentary sovereignty] has never before recognised as law the abstract trick of the legislature taking over the executive power and passing it off as a law of the nation.” However, the law is ultimately what Parliament declares it to be, and courts cannot invalidate an Act of Parliament just because it doesn’t comport with past practice.
 The ecclesiastical courts could grant a separation, but not a divorce. The main difference being that separated couples could not remarry while their former partner still lived, but divorced couples could.
 Seven such Acts have been passed since 1975, the most recent being in 1987.