This article by Joshua Rozenberg gives a very clear and objective summary of the Brexit appeal which begins next week in the Supreme Court. He not only looks at the issues which the Court has to decide but also the composition of the Court and the procedure which will be followed.
The High Court defined prerogative powers as “the residue of legal authority left in the hands of the Crown”. As the Queen always acts on the advice of her ministers, “the Crown” in this context means executive government headed by Theresa May and the Cabinet. Because the UK’s constitution is not written down in a single document, there is no authoritative list of the Crown’s powers. But prerogative powers are limited to those that have previously been recognised by the courts. They cannot be extended. And, because parliament is sovereign, prerogative powers can never be used to overrule legislation. Unless ministers can rely on valid prerogative powers, they cannot change the law without the authority of parliament. The specific prerogative power that the government says it can rely on in these circumstances is the power to “make and unmake treaties”.