In 1688 there was a prat called James II on the throne. His elder brother, Charles II was probably the best of the Stuart Kings because he concentrated more on the Boudoir than interfering with the country. Indeed on one of his nightly forays his younger brother had suggested he shouldn't go out into dangerous streets with so little escort, to which he allegedly replied: “There is not a man in all my kingdom who would kill me to put you on the throne”.
And so it proved to be when King James II brutally crushed the Monmouth Rebellion and got himself thoroughly disliked for wanton abuses of power. In 1688 a group organized a 'convocation' of all the worthies of the Realm and told James II to hop it. They also invited the Duke of Orange to become the first Constitutional Monarch. This means that although the Sovereign may not be over-ruled, the definition of Sovereign, they are, nonetheless, bound by limits to their power.
Chief among the strictures imposed was the imperative that the Nation may not be given away so no power could fine us a Nation, nor could they over-rule our courts. At the time the worry was the Catholic hegemony, which having lost Britain a 150 or so years earlier, was still scheming to get a puppet on the throne.
Although Cromwell's Commonwealth was within living memory, and they were dumping a pretty rotten Royal, they nevertheless chose the Monarchy over a Republic. They also put all authority within the hands of the Sovereign to be lent out on the receipt, directly or indirectly, of a bondsman's oath. This means judges, the police, the armed services et al. have no authority in their own right, but rather the authority the sovereign lends them.
Thus they effectively put all their eggs in one basket and made the protection of the sovereign and national sovereignty paramount. With hindsight the reason is clear, a sovereign, being a person, may be held accountable for the actions done in their name, and we can all agree accountability is sadly missing today.
Lawyers will try and persuade you that since GB has no written Constitution it has no Constitution. This is not so, we have 'Constitutional Documents' of which by far the most important are the Magna Carta and the Declaration of Rights 1688, the result of James II demise. The reason these carry more weight is that in both cases the Rule of Law had broken down and the documents represented the conditions under which the rebels would give their Consent to the Rule of Law.
This is the purpose of a Constitution, to set bounds on The Powers That Be, to draw the line beyond which the people will withdraw consent and rebel. TPTB then try to use the Authority conceded to them to 'adjust' the Constitution bit by bit, snipping away at Liberties. Americans are vigilant on this front, but in Britain TPTB have gradually rewritten the Rule Book, or at least they think they have, and claim our implied Consent by the fact we haven't rebelled... yet.
So the Constitution is not an outdated legal thing with little relevance today, as the TPTB will have you believe, rather it is that line beyond which lies rebellion, withdrawal of Consent to The Rule of Law. A constitution cannot be decreed for us any more than respect can be demanded, yet it is respect for the law that leads to consent to the rule of law.