'That the freedom of speech and debates or proceedings in Parliament ought not to be impeached or questioned in any court or place out of Parliament'
It is often said that Britain does not have a written constitution. In fact, Britain does not have a codified constitution, which means that it is not written down in a single document. Parts of the British constitution are indeed in writing, and one of principal pieces of legislation forming the constitution is the Bill of Rights 1688. (This is the name given to it in the Government website; it should be called the Bill of Rights 1689, but let's not quibble.)
Prorogation is a 'proceeding in Parliament'. It ought, therefore, not be questioned by any court, including the Supreme Court.
I suggest that the Supreme Court has overstepped its powers and should be abolished and replaced by the previous system of law lords.
So, who were the law lords? Their official name was the Lords of Appeal in Ordinary, and, as a result of the Appellate Jurisdiction Act 1876, and their function was to act as the highest court of appeal within the United Kingdom.
Most the law lords were long-serving barristers or judges who were given peerages by the Government of the time. Over time, the number of law lords was increased. Each Government could appoint new law lords, usually on retirement of incumbents, especially if they believed that there was a political imbalance amongst the law lords.
The thinking behind the establishment of the Supreme Court was that law lords had both judicial and legislative responsibilities. There was a desire for a body that would be separate from the House of Lords and would have only judicial responsibilities. There was the possibility that a peer could have judicial, legislative and executive roles, and, indeed, this was true of the Lord Chancellor until 2005.
I do not see a problem with this weak separation of powers at levels lower than the monarch, who heads all three powers.
Much of the thinking behind the establishment of the Supreme Court was not about separation of powers. Rather, it was to establish a court in Britain that would have the same role as the European courts.
One of the major problems with the Supreme Court is that it appoints its own members. There is therefore a strong possibility that the court may become biased in a single direction. This is what caused the court first to consider the issue of Boris Johnson's prorogation of Parliament and second to rule that the prorogation was unlawful. The court should not have considered the case at all.
Following Brexit, I would expect the next Conservative Government to remove this unconstitutional court and revert to the system of law lords.