The British Isles consist of several kingdoms, large and small, constantly vying for ultimate control of the region as a whole. Of note here is that a kingdom can be much smaller and less influential than what we think of in the modern day as a kingdom. Indeed, many kings were little more than petty earls or dwarfed by a neighboring duchy. As well, there were different ranks of king; as the campaign progresses both Uther and Arthur receive fealty to several smaller, less important kings with no change in either's title or standing. Logres was the most important with the kingdoms of Cornwall, Malahaut, Nohaut, Norgales, Estregales, Wessex and Essex as much smaller entitites
Logres is the largest and most important of the island kingdoms. It comprises the bulk of what we know of in modern times as England. The Pendragons are from Logres and it has always been the axis around which the rest of the Isles revolve. Rome's seat of power was in Logres and London remains the largest and most important city in Western Europe outside of Paris.
About seventy years prior to the start of the campaign the Supreme Collegium or High Council of Britain chose a native leader, known as Emperor Constant, to rule the island. He was a fair ruler and in 440 his son, Constans, succeeded him. A few years later, Constans was killed and the throne was assumed by a man named Vortigern
At the time of Vortigerns accession, a massive Pictish and Irish invasion threatened the entire island until Vortigern hired a small mercenary army of Saxon warriors. Victory was not enough, though, and, inflamed by lust, Vortigern married the daughter of the Saxon King Hengest. Vortigern also brought Cymric leaders from the warlike North and settled them around the western coasts to defend against the Irish raiders. Alas, Vortigern proved exceedingly tyrannical and was not well loved
Soon, many eastern lords of Britain rebelled against their new king only to be crushed and have their lands given to Vortigern's Saxon allies. Vortigern's own son led a second rebellion in the 450s only to to be killed in battle. Then, during the infamous Night of the Long Knives, the Saxons betrayed Vortigern, slaying nearly all the British leadership