In R (Miller) v Secretary of State for Exiting the European Union,  UKSC 5, the UK Supreme Court holds that the approval of the UK Parliament, but not ― as a matter of law anyway ― of the “devolved” legislatures of Scotland, Northern Ireland, and Wales is required before the UK government can serve notice of its intention to leave the European Union. Mark Elliott has already posted a full and, to me, a largely compelling critique of the decision on his (excellent) blog, Public Law for Everyone. The Judicial Power Project has posted shorter comments by eminent public lawyers, including John Finnis and Timothy Endicott. They and others say most of what there is to say about Miller, but I want to take note of its treatment of one specific issue, that of constitutional conventions, on which I part ways both with the Court and with the commentators who, however critical they are of its reasoning on other points, follow conventional wisdom on this one.
The Miller Court is perfectly orthodox on this point, reaffirming the Diceyan distinction between law and convention, the former being justiciable and the latter not. In my view, the Court is wrong to do so. Its reasoning on this point shows that the line which it attempts to draw between law and convention is so thin as to be evanescent. Indeed, it is at least arguable that its reasoning on the main issue, that of the availability of the royal prerogative to trigger the UK’s withdrawal from the EU, is in direct contradiction with that which underpins its refusal to treat conventions as legal, judicial cognizable rules.
As the majority judgment explains, one of the issues in Miller concerned the effect of the so-called Sewel Convention, which
was adopted as a means of establishing cooperative relationships between the UK Parliament and the devolved institutions, where there were overlapping legislative competences. In each of the devolution settlements the UK Parliament has preserved its right to legislate on matters which are within the competence of the devolved legislature. 
However, from the outset, there was an expectation that, as a matter of convention, the UK Parliament “would not normally legislate with regard to devolved matters” without the consent of the affected devolved legislature. “That expectation has been fulfilled,” says the majority.  It has been embodied in “memoranda of understanding” between the UK government and devolved authorities, and more recently in a statutory provision, section 2 of the Scotland Act 2016, which “recognised” the convention.
For the Court, none of that meant that it could pronounce on the applicability of the Sewel Convention to the matter at hand ― that is to say, on whether the convention required the UK government to seek the devolved legislatures’ consent before seeking to withdraw from the EU ― or indeed to any other issue. That is because “[i]t is well established that the courts of law cannot enforce a political convention.”  The quoted at length from the various opinions in the Patriation Reference, Re: Resolution to amend the Constitution,  1 SCR 753, finding there support for its view that the political and the legal are distinct realms, and that while courts “can recognise the operation of a political convention in the context of deciding a legal question … they cannot give legal rulings on its operation or scope, because those matters are determined within the political world.” 
But why is there this impenetrable barrier between the legal and the political? The majority’s explanations are sparse, to put the matter rather generously. In addition to the quotations from the Patriation Reference, we are told that “[j]udges … are neither the parents nor the guardians of political conventions; they are merely observers”,  and directed to Colin Munro’s assertion that “the validity of conventions cannot be the subject of proceedings in a court of law” (“Laws and Conventions Distinguished” (1975) 91 Law Quarterly Review 218 at 228″).
Munro’s words, at least, have been flatly contradicted by events ― namely, by the Patriation Reference itself, as well as by the other cases in which the Supreme Court of Canada and other Canadian courts have pronounced on the “validity” of alleged conventions: notably Re: Objection by Quebec to a Resolution to amend the Constitution,  2 SCR 793 (a.k.a. the Québec Veto Reference), Public School Boards’ Assn. of Alberta v. Alberta (Attorney General), 2000 SCC 45,  2 SCR 409, and Ontario English Catholic Teachers’ Assn. v. Ontario (Attorney General), 2001 SCC 15,  1 S.C.R. 470. Contrary to Munro’s assertion (and Dicey’s stated belief that conventions were matters of such exalted political importance that they were “too high” for mere lawyers ― a belief contradicted by his own magisterial treatment of the subject!), courts can deal with conventional questions.
Indeed, it seems to me that the Miller majority is less forthright about this than it really ought to have been. In introducing one of the quotations from the Patriation Reference, the majority describes it as being from “a dissenting judgment on one of the questions before the court”.  It does not say what question. So let me remind the reader: that’s the question of whether a convention prevented the federal government from seeking Patriation without provincial consent. The majority knows this, of course, and thinks it better not to be explicit.
Whatever the merits of this rhetorical approach, with Munro’s impossibility assertion out of the way, what is left is the Miller majority’s argument is the the claim that courts should not deal with conventions because, due to their political nature, the courts are not their “parents” or “guardians”. This echoes the position of the Patriation Reference‘s majority on the legal question that conventions are “political in inception” and thus by their “very nature” incapable of “legal enforcement”. (774-75) But this too, is not much of an argument. Statutes too are “political in inception”, yet courts enforce them ― as “guardians”, in the Miller majority’s terminology. Of course, conventions often look less like statutes than like common law rules, in that they lack a well-defined authoritative formulation ― though this is not true of the Sewel convention, which has been in fact authoritatively, if somewhat vaguely, stated for as long as it has existed. But even we take the analogy to common law rules, what is it that stops courts from being “parents”, or perhaps adoptive parents, to new common law rules into which conventions crystallize?
In the Patriation Reference, the legal question majority had to address this contention:
The leap from convention to law is explained almost as if there was a common law of constitutional law, but originating in political practice. That is simply not so. What is desirable as a political limitation does not translate into a legal limitation, without expression in imperative constitutional text or statute. (784)
This response is bizarre, in that there obviously is a “common law of constitutional law”, including the rules on the Royal prerogative at issue in Miller, as the Patriation Reference majority well knew. Is the suggestion that that law did not “originate in political practice”? But what exactly did it “originate in”? Did the judges ― say Coke in the Case of Proclamations ― simply make it up, or pluck it out of thin air?
Whatever the view of the Patriation Reference majority, the Miller majority is not entitled to its predecessor’s claim that “[w]hat is desirable as a political limitation does not translate into a legal limitation, without expression in imperative constitutional text or statute.” Its decision on the main issue in the case rests in part on its view that “[i]t would be inconsistent with long-standing and fundamental principle for … a far-reaching change to the UK constitutional arrangements to be brought about by ministerial decision or ministerial action alone,” without Parliamentary authorization.  This principle is not, needless to say, to be found “in imperative constitutional text or statute”. Longstanding or not, it is a view of “what is desirable as a political limitation” ― and, according to the Miller majority, it does “translate into a legal limitation” on the UK government’s powers. (To be clear: this is not the entire basis for the majority’s decision; but it is a important part of its reasoning.)
The belief that there is a fundamental difference in the nature of legal and conventional constitutional rules never rested on much of anything other than the assertions of scholars and, eventually, courts that have uncritically followed these scholars. The distinctions that they have attempted to draw between law and convention do not involve material differences. Ironically, the Miller majority’s own reasons strongly suggest as much. When it considers the effect of the “recognition” of the Sewel convention by the Scotland Act 2016, it concludes by incorporating it into statute,
the UK Parliament is not seeking to convert the Sewel Convention into a rule which can be interpreted, let alone enforced, by the courts; rather, it is recognising the convention for what it is, namely a political convention. 
This seems to me to acknowledge that the source of a rule ― statute or convention, or in another case the common law ― is less material than “the nature of the content”  of that rule. Some rules, whether ostensibly legal or conventional, do not let themselves to judicial interpretation or enforcement. (Whether it is the case that the Sewel convention is such a rule is a separate question which I will not try answering here.) But other rules do lend themselves to judicial interpretation or enforcement ― and for them too, it should not matter whether these are ostensibly legal or conventional rules. The question the court ought to have asked itself is whether the rule is suitable for judicial application ― not whether it is law or convention.
My views on the distinction, or lack thereof, between law and convention (which I have sought to explain at greater length in my paper “Towards a Jurisprudence of Constitutional Conventions”, (2011) 11 OUCLJ 29, and briefly in a forthcoming piece in the Supreme Court Law Review) are, I am well aware, rather heretical. Yet to me the conventional thinking on the issue of constitutional conventions, and conventional arguments for distinguishing them from legal rules, are simply not convincing.