Would this use of *nobile officium* be a separation of powers violation?

Question

I've mentioned here and here that certain Brexit-pursuing actions as Prime Minister, by Theresa May and Boris Johnson, raised separation of powers concerns quite separate from the policy question of whether/how Brexit should occur - specifically, apparent cases of executive overreach.

A recent legal challenge seeks to use nobile officium to ensure that, if Johnson refuses to send an extension-requesting letter to the EU if compelled to do so on October 19 2019 by the Benn Act, the Court of Session could send one with the same executive authority as if he had. My question concerns whether this would also be a separation of powers violation. Before I explain why I'm asking, though, I'll mention nobile officium is defined as

The noble office or duty of the Court of Session. An equitable jurisdiction in virtue of which the court may, within limits, mitigate the strictness of the law and provide a legal remedy where none exists.

One description of the reasoning behind such a usage is that, because the Benn Act would compel the executive to act a certain way on October 19 (if the October 17-18 European Council summit doesn't secure a Brexit deal), it deprives the Prime Minister of certain executive powers, and those powers may as well be exerted by someone else, such as the above court. In the words of Phil Moorhouse,

The Benn Act removes any executive decision-making from the executive, so, you know, the suggestion is, well, we don't actually need the Prime Minister, then, do we, if he refuses to sign it? I mean, we'd like him to sign it, that would be much tidier; but if he refuses to sign it, well, he's compelled to anyway. It's got nothing to do with him. He doesn't have the power to refuse because Parliament have legislated for it. So in this scenario, can we just put a signature down there that explains the situation and send it off? And hopefully, that's good enough for the EU council.

I don't want my opposition to a no-deal Brexit (well, Brexit in general if I'm honest) to bias me in favour of (ab)uses of the three powers that serve my interests and against those that undercut them; I want to consistently defend the separation of powers as a component of democracy at least as important as elections. So the following thoughts occur to me:

1. Does the executive-stymieing effect of the Benn Act make it a case of legislative overreach?
2. Even if it doesn't, would a court "impersonating" No. 10 constitute a case of judicial overreach?

I am not a constitutional lawyer (well, I'm not a lawyer at all), so I don't know what theory or precedent, in any nation, has to say about this. But my gut feeling is that negative answers may be available to each of these questions, which would go something like this:

1. This Parliament has seen executive overreach that is historically unprecedented, or close to it, and there are genuine reasons to suspect Johnson might try to circumvent Parliament yet again, so the Benn Act may be not so much an offensive legislative overreach as a defence against a predicted executive overreach. One may also argue this as part of Parliamentary sovereignty (I say, with little understanding of the limits on it); this sovereignty was upheld in the Supreme Court's recent voiding of Johnson proroguing Parliament.
2. If the Benn Act successfully changes, in a not unconstitutional way, who has this particular power, the judiciary is empowered to manage any executive-legislative dispute this creates, even if this means the power either being transferred again, or a change in who sends the letter on behalf of a Parliament legislating its authorship.

But what do people who know better than me think about it?

Answer 1

Historically, the concept of "legislative overreach", and indeed the concept of separation of powers had no strong place in UK law. Indeed until 2005 the Lord Chancellor was a senior figure in all three of Montesquieu's branches, presiding over the House of Lords, being a member of the Cabinet and sitting as a judge in a number of roles, in what was sometimes termed a fusion of powers.

Further the fact that the executive was drawn from the legislature, and was generally expected to command a majority within the House of Commons squared the circle with the concept of Parliamentary sovereignty, that is, the idea that Parliament could pass any law it chose (except one irrevocably binding a future Parliament), with courts unable to do anything except interpret the rules handed to them. As such, it is partly the relatively long period of coalition, weak majority and minority governments which have stressed the system enough for the executive to repeatedly act in manners which have been found unlawful.

Somewhat ironically, some of the biggest opponents of judicial influence on law making (through bodies like the European Court of Human Rights) are also those currently upset at Parliament choosing to pass unusual laws, some of which greatly impact on (notional) prerogatives of the Crown (and thus the executive). To a greater or lesser degree, we live in interesting times.

Answer 2

I'm not sure this is entirely worthy of a separate answer, but nobody seems to have pointed out on this page that nobile officium is apparently only accepted in Scots law. If the challenge using nobile officium to the PM (not signing the extension letter according to the Benn Act) is successful in the Court of Session (i.e. the Scottish supreme court), it will would probably not sit easy with the rest of the UK, which insofar doesn't seem to have much acceptance of this nobile officium principle. So there will probably be some UK-wide constitutional tension, perhaps to be resolved by the UK Supreme Court.

Note that the power of nobile officium rests with the Inner House of the Court of Session, and its decisions can be appealed to the UK Supreme Court.

The Inner House is the senior part of the Court of Session, and is both a court of appeal and a court of first instance. The Inner House has historically been the main locus of an extraordinary equitable power called the nobile officium – the High Court of Justiciary has a similar power in criminal cases. [...]

Unlike in the High Court of Justiciary, there is a right of appeal to the Supreme Court of the United Kingdom of cases from the Inner House.