The Court of Session’s verdict that prorogation is unlawful is a major headache for Boris Johnson. It makes the Supreme Court’s decision on the matter, and the court will hear the case on Tuesday, much more unpredictable. There is now a significant chance that parliament will have to be recalled.
The Supreme Court will hear all the various cases on prorogation at the same time on Tuesday, remember the government won in the High Court in London. But it will adjudicate these cases in line with the law of the court from which they came. So, it will decide the case from the Court of Session according to Scottish Law and the one from the High Court according to English law. This raises the possibility of them deciding it is legal under English law but not Scots. I understand that in these circumstances, parliament would have to be brought back on the basis that the prorogation would have to be lawful in all parts of the UK.
If parliament does end up returning early, it would make life very difficult for Boris Johnson. It has, of course, already passed legislation requiring the Prime Minister to seek an extension if no deal has been reached by October 19th. But convinced of its own righteousness, it would use this extra time to go on various fishing expeditions—demanding the messages of various aides and more no deal planning documents. Rather than being able to use the Prime Ministerial bully pulpit to command the news agenda, Boris Johnson would find himself being defeated in parliament on an almost daily basis.