Dr Catterall wrote: “Government policy may not be law, but that policy is still subject to the rule of law and has to operate within the law. In other words, policy can only operate where it is legally entitled to do so: governments cannot choose to do whatever they wish to, however popular such actions might be, if they do not have the legal title to do so.”
He added: “The implication of the ruling was […] that explicit parliamentary approval was needed to trigger Art. 50. Since it is clear that parliament will give such approval, you might wonder why the government immediately appealed to the Supreme Court.”
After analysing a series of issues at stake in the case, Dr Catterall concluded: "All of this points to a problem central to the Miller case. Art. 50 of the Lisbon Treaty allows an EU Member State to ‘withdraw from the Union in accordance with its own constitutional requirements’. For most of the other 27 Member States this is a less contentious issue. For the UK, with a relatively uncodified constitution and a devolution settlement which is still evolving - not least given that the implications of the 2016 Scotland Act have yet to be tested in the courts - this is certainly not the case.
“These constitutional questions, rather than Brexit, are really what is at issue in this instance. The Supreme Court is in essence being asked to make a judgement on principles that have far wider consequences than the ostensible bones of contention in Miller. In the High Court, the Lord Chief Justice and his colleagues effectively got themselves out of this dilemma by a ruling of constitutional conservatism, in favour of the continuing centrality of parliamentary sovereignty to the UK constitution.
“The Supreme Court may not find it easy to get to a similar point given the complicating factor of the devolution issues they have been asked to consider. However, when they deliver their ruling in early 2017, notwithstanding the risk of being pilloried by the Daily Mail, they may well find that a judgement which affects as little as possible conventional understandings of those constitutional requirements is the one which opens the fewest cans of worms.”