I know I'm way out of my depth here, so don't expect any contributions from me: just questions.
It could be argued that the various governments of the British Isles go back pretty much as far as any. There was a significant change in 1066 after the Normans conquered England, but although William I introduced Norman customs, this was merely a case of one king replacing another. Of course there have been many changes - some profound, such as the Battle of Hastings already alluded to, but (apart from a few minor gaps) there has been a King of England since 827, whether he was Saxon, Dane, Norman, Welshman, Scot, or German. You have to go back a long way to find a truly English king, and maybe you can't, but that's by-the-by. Then you have Runnymede, where, in 1215, the English barons demanded and obtained restoration of ancient privileges. A change in the balance of power, but the King remained the ruler of the country, although Parliament now had a say in matters of taxation and justice. And gradually that power shift continued until a great struggle for superiority developed in the 2nd quarter of the 17th century, and eventually the Civil War led to the executuion of the King Charles I and the country was ruled by Oliver Cromwell under the Protectorate. That was a significant change: England became a republic, but this was shsort-lived, and when Cromwell's son proved an inept ruler, the monarchy was restored in 1660. So England was restored to a kingdom, and the House of Stuart resumed the throne in the person of Charles II. So the changes brought about by the Civil War did not change the system of government authorities, although it brought them under Parliamentary control. Just another shift or power.
After that, the evolution of England's sytem of government consisted primarily of granting the right to vote to more and more people. In 1689, by the Bill of Rights gave people rights through their representatives in Parliament. Then in 1832 the Reform Bill gave seats to counties or large towns, and gave vote to householders worth 10 pounds A further reform bill in 1867 gave the vote to country laborers and in 1918 the Representation of the People Act gave the right to vote to the women in Britain. By 1928, universal suffrage for adults obtained in Britain, bu the original instituions, created centuries before, still remain and still function, albeit in a very different manner than originally. Is this an argument for dating the English system back to 827?
In Scotland, King Kenneth Mac Alpin became king of the Picts and the Scots (=Irish). This left the Britons (related to the Welsh), the Angles (from Europe via England) and the Norse, but they all eventually became absorbed into the Kingdom. When the country wasn't fighting its routine wars with England, the aristocracy was busy murdering each other in an attempt to jostle themselves into power, and without meanng to denigrate the history of such a noble country, or to make slight of it, not much else happened until the Union of the Crowns in 1603 led to King James VI of Scotland becoming also King James I of England. Scotland's Parliament voted itself out of existence, but its Parliamentatirans simply moved to London and enacted Scotland's laws from there, with English help and guidance. The rest of Scotland's Establishment remain. The argument that Scotland's government has continued since 842 is less tenable than the case for England, but it can be made.
A case could also be made for the Isle of Man. Its government - the Tywald - goes back to ancient times, although Mann has been under the domination of, variously, Sweden, Scotland, Ireland and England. More than that, however, I do not know.
Looking over to the east, how old are the Tibetan and Thai systems of government, I wonder. And to the west, there are the Indian Nations which are, I believe, technically independent countries. How old are those tribal governments?