The Supreme Court judges were unanimously agreed that proroguing Parliament was illegal but their decision was not based on law or precedent nor did the most senior judges in England (and Wales) agree with them.
When the Supreme Court’s judges ruled that the Prime Minister’s prorogation of Parliament was illegal Remainers delightedly announced that it was not just stupid but supremely undemocratic. The previous English High Court judges (The Lord Chief Justice, The Master of the Rolls and The President of the Queen’s Bench) had earlier ruled that the matter was not for the courts to decide and this was the advice the PM had been given by Attorney General, Geoffrey Cox. Despite the unanimity of the eleven judges it was therefore not clear cut as other very senior lawyers disagreed with them.
We thought at the time  that the decision to prorogue was motivated by political considerations, to defy the twisting and bending of rules and conventions that Parliament (in particular the Speaker) had used but which the Government had not put before the courts – well it couldn’t because there is a clear statute preventing the courts from interfering with Parliament. It was obvious that the PM had Brexit in mind and not just a Queen’s speech, a week usually serves for that. The Government argued that there would anyway be a recess for party conferences but Parliament could have cancelled that (as it now has, for the Conservatives).
Nevertheless UK law seems to have taken a very EUropean direction . It used to be based on precedent to ensure that ‘everyone is equal before the law’, a literal reading of formal evidence rather than a subjective one, and the concept that ‘everything is legal except that which is forbidden’. Now the British constitution is open to semantic interpretation and appointed judges decide cases by reading what intention lay behind the actions of the accused, a very Continental approach to the law (i.e. it moves the UK closer to EU norms).
This is a dangerous precedent itself; suppose for example that an appellant had asked judges to decide what was David Cameron’s intention in calling a Referendum on EU membership. Was it because he wanted to give the people the right to decide what they thought was best for their future or was it to save the Conservative Party despite what might be in the best interests of the country? Plenty of those in favour of remaining in the EU believe the latter interpretation while those who wanted to leave the Union welcomed the chance to instruct him accordingly. It was a political question that judges would surely have refused to engage with.
The Supreme Court judges’ view of what the Government wanted to achieve is very probably true, as many committed leavers would agree: to prevent further devious actions by Parliament to prevent the Government from governing or appealing to voters for their support. Precedent shows that prorogation has often been used for political advantage. John Major, one of the appellants in this case, is suspected of using it to delay the release of a report into ‘cash for questions’ (in which two Conservative MPs were accused of taking payments to put questions before the House) so that he could call the 1997 election before the facts were revealed—he lost the election anyway. David Starkey, the constitutional historian, has pointed out that Clement Atlee used prorogation to prevent the Lords delaying his major nationalisation programme. In 1948 he prorogued Parliament twice in quick succession to prevent the Upper House delaying his bills beyond the life of that Parliament and a possible change of government, surely that is a clear precedent.
Skulduggery in Government and Parliament is commonplace but the convention is to throw out both if they misbehave and ask the people to decide who should rule the country. What the anti-Government brigade in Parliament intend is not to offer voters the chance to rethink their 2016 decision in the light of what they have seen and learnt since but to exclude the Leave option and replace it with one that ties the UK into EU Treaty regulations that may also change without any representation from the UK. This is outrageous, we wonder whether the judges would agree.
Johnson did not break the law because there was no law on prorogation when he did it, the Supreme Court invented the law after the event so the outrage in Parliament from his opponents was just theatrical politics. In fact the whole case was pure politics in which the Court should not have intervened. In the judgement announced by Lady Hale these were defined as “exceptional circumstances” which is clearly a political assessment. The law says we must leave on 31st October and also that we cannot leave until at least 31st January provided a foreign power, the EU, agrees. John Major prorogued Parliament for three weeks so that’s OK but five weeks is too long, so what’s the limit (the Court didn’t say)?
 Where are we Sailing? (Proroguing Parliament)