Organically rather than by design, laws and courts preceded the theoretical constitutional basis. Kings developed a practice of legislating in council, rather than solo, meaning that their edicts went out not as "the King decrees XYZ" but "the King, with the assent of the archbishops, bishops, abbots, barons, etc., decrees XYZ". This eventually became the Parliamentary system, wherein only Parliament could make laws (especially about raising taxes) and the King alone could not.
Because of the gradual evolution, it's not easy to point to the first such statute or even the first proper "Parliament". The form was not settled. Nowadays we expect Acts to start with a set "enacting formula", to be published in a set way, and to be called "Acts" at all. Earlier laws lacked these elements. It was common for Parliaments to transact a variety of business, such as appointing officials or distributing money, without strictly distinguishing a category of what we would now call a "public general Act", something setting out broadly applicable rules under the tenor of law. To take an early example, the Domesday Book was undoubtedly commissioned by William I in council in 1085, but we could argue for a long time whether that really counted as a law made by the King alone or through a proto-parliamentary approach. The Assize of Clarendon in 1166 is closer - we have the text and it refers to the counsel of all the barons - but still not obviously "an Act". Magna Carta came in 1215 and was repeatedly confirmed by successive monarchs in slight variations; only the 1297 version is currently deemed to be an Act still partially in force.
At the same time as this legislative activity, a bewildering array of courts applied essentially customary law for local cases. If you commit a murder in Durham in 1300 and would like to not be executed, then you would be appealing (I think?) to the mercy of the Prince-Bishop in his own court. A few dozen miles away and the process and authority was different.
Things gradually became more uniform and systematized, and by the sixteenth century there was enough critical mass of legal philosophy to try to "explain" the legal system more formally and in recognizably modern terms. We may as well take Edward Coke's Institutes of the Laws of England (1628-1644) as a turning point: four volumes setting out a comprehensive view of what English law was all about. In contrast, take Bracton's similar work of 1235, which was the first real analysis of case law in England. Bracton was writing in a context without a general idea of precedent, and where the binding extent of Roman civil law was not clear. By Coke's time that was well established. But they are both trying to make sense of the law as they found it: giving explanations for the data, if you like. (A parallel course was taking place in Scotland with its institutional writers, although they ended up with a different system from England's. They were equally trying to explain what they saw.)
Some of the early modern authors contributed to figuring out which decisions of past Parliaments or councils should be taken to be Acts, as such. Coke, in particular, also set out the criterion that an Act should purport to have been made by all three of the King, Lords and Commons. Our modern consolidated versions of the statute book derive from this time, and on legislation.gov.uk you can see the oldest Act still in force, The Statute of Marlborough 1267. That's different from the oldest Acts ever made, as discussed above, a more slippery concept.
Ideas such as the separation of powers are reverse-engineered in this way. Indeed, it's not obvious that Britain has that separation. It may not be the right way to understand the governmental and legal order that has developed. John Locke might have thought it would be a good idea, but Walter Bagehot denied it was a useful frame for analyzing the interrelationships of various public bodies.
