3.2.2 Judicial deference
The principle of judicial deference refers to the extent to which the judiciary ought to defer to the sovereignty and legitimacy of Parliament when coming to judgments. Legal academic Aileen Kavanagh gives the following definition:
… judicial deference occurs when judges assign varying degrees of weight to the judgments of the elected branches, out of respect for their superior expertise, competence or democratic legitimacy …
The reasons that Kavanagh gives for adhering to the principle of judicial deference are that Parliament is thought to be more legitimate than the courts because it is democratically elected, more expert than the judiciary in matters of policy, and more competent than lawyers (who are trained only and specifically in legal rules and interpretation) to turn policy into law. Deference is a principle applied by the courts, especially when dealing with sensitive political issues – such as national security – that are often considered best left to the other branches of government. The principle is controversial, with some academic commentators believing that the judiciary is too deferent to Parliament and other commentators considering that it ought to maintain deference. Understandably, it is often politicians within the government of the time that argue most stridently that the judiciary should be more deferent to the wishes of Parliament than it currently is. This has become increasingly so with debates surrounding the expanding scope of human rights law..
There have also been a number of high-profile cases testing the relationship between judiciary, executive and legislature surrounding decisions taken as part of the process of exiting the EU. These include the Article 50 case (R (on the application of Miller) v Secretary of State for Exiting the European Union  UKSC 5 ) and the case relating to the prorogation of the UK Parliament (R (on the application of Miller) (Appellant) v The Prime Minister (Respondent) Cherry and others (Respondents) v Advocate General for Scotland (Appellant) (Scotland)  UKSC 41A). The latter case is of particular interest as decisions in the lower courts of England and Scotland differed. A House of Commons library insight notes:
On 11 September, the High Court of England and Wales held that the legality of the prorogation was not justiciable in a court of law. That meant that the High Court had determined the question to be beyond the scope of judicial review. On the same day, the Court of Session in Scotland reached the opposite conclusion. It determined that the issue was justiciable. It then concluded that the decision to prorogue Parliament was motivated by an improper purpose.
The Court of Session concluded that the prorogation was “unlawful and thus null and of no effect”.
The case went on to be heard by the UK Supreme Court which, in relation to the justiciability aspects of the case, held that the power to prorogue the UK Parliament cannot be unlimited and must, therefore be subject to judicial review.