Could Parliament still block Brexit? The answer is undoubtedly Yes. On 3 November 2016 the High Court decided that Theresa May needs Parliament's consent before triggering Article 50 of the Lisbon Treaty to leave the EU.
The Sovereignty of Parliament
The point is that the referendum was purely advisory. This cannot be disputed, because of that most fundamental principle of the UK Constitution, the Sovereignty of Parliament - which I stressed in my last two blogs and which was the basis of the exemplary decision of the High Court, but which was completely lost sight of by the Government as well as by 1,054 barristers in an open letter to David Cameron.
Could the referendum result have been made binding? Yes, very easily. The original referendums on Scottish and Welsh devolution in 1979 were binding. In both cases an Act of Parliament was passed in 1978, before the referendum was held, which actually set up devolved assemblies for Scotland and Wales. The 1979 Welsh referendum failed dismally, with only a 20.3% vote in favour of devolution, so the 1978 Act was repealed as it itself provided. The 1979 Scottish referendum also failed, but much more narrowly. There was a 51.6% majority in favour of devolution, but this was less than 40% of those entitled to vote, so, according to the provisions of the 1978 Act, the Act was repealed, and Scottish devolution had to wait until a fresh referendum was held in 1997.
But making a referendum result binding can be done much more simply. A referendum can only be called by an Act of Parliament. The 2016 EU referendum was called by the EU Referendum Act 2015, which says absolutely nothing at all about what is to happen after the referendum. Could that Act have delegated to the Government the right to trigger Article 50 in the event of a "Leave" majority? Yes it could. But the crucial point is that it didn't.
As a result, the EU referendum was purely advisory - meaning that it advised Parliament, giving Parliament the right to accept that advice (with or without conditions) or to reject it. If allowed to, Parliament will almost certainly accept the voters' Brexit verdict, but it will probably want to impose some conditions on the Government, which it is perfectly entitled to do.
What are the alternatives before the Supreme Court? They are:
a. To affirm the High Court's ruling that Parliament's consent is needed to trigger Article 50. This is undoubtedly the best option from both a legal and a political point of view. Parliamentary consent must be given in the form of legislation.
b. To overturn the High Court decision and allow the Government to trigger Article 50 without Parliament's consent. Because of the Sovereignty of Parliament, this would be wrong. The Government's elaborate arguments in court about the power of the royal prerogative over treaties were largely irrelevant.
c. To require parliamentary consent to triggering Article 50 subject to certain specific conditions. This would not be a good option for the court to select, as any conditions are a matter for Parliament. The court should confine itself to deciding whether parliamentary consent is needed to invoke Article 50 or not.
d. To allow the Government to trigger Article 50 without parliamentary consent but subject to certain other conditions. This would also not be a good choice, as this would amount to the court's taking charge of the Brexit negotiations, which is a government function.
The elephant in the room is the position of the devolved governments of Scotland, Northern Ireland and Wales. The Scottish Government is particularly insistent that Scotland should not be dragged screaming out of the EU. But neither the Sewel Convention of 1998 nor the Declaration of Arbroath of 1320 has the force of law.
The best solution would be for the Supreme Court simply to affirm the decision of the High Court requiring Parliament's consent (by way of legislation) for Brexit. Parliament could then impose whatever conditions it wants.