According to their national myth, the ancient Britons were descendants from the Romano-Trojan Brutus, who, with the aid of the Goddess Diana, journeyed to the isle of Albion and subjugated the natives. Brutus named the island Britain after himself, and he and his fellows proved to be enlightened rulers, establishing towns, cities and the rule of law. His successors included the legendary King Lucius, Britain's first Christian King and most famous of all, the warrior king Arthur Pendragon. King Arthur had slaughtered the Saxons, pacified the Picts, subdued the Scots, impaled the Irish and even routed the Romans. However the Angles were one enemy too many, and Arthur had to retreat to the isle of Avalon in order to recoup and heal his wounds. The Britons had been defeated, but the wizard Merlin swore that one day Arthur would return, and that Britain would be Briton once more...
In our timeline, the ancient Britons never did reconquer Britain. Instead they adopted a sort of defeatism and separatism from the rest of Britain which to some extent, exists to this day. Whereas the invaders turned outward and assimilated, becoming English and Scots, the Britons turned inward, and even adopted the labels of their conquerors: Wallia, the border people. The ideal of Brytanyets (or Britons) was replaced by that of Cymry (or "Compatriots") and even the legend of King Arthur was forgotten, only to be readopted by the English invaders as their own some years later. With such a defeatist outlook, it was only a matter of time before the Britons would be themselves subjugated by the English...
But what if someone had made a stand? What if the Britons turned outwards instead of inwards? What if they developed a sense of "manifest destiny", took up the sword and longbow and reconquered the lands to which are theirs by right? What if they had a leader, a man of justice, strength and strong will who was capable of guiding his people towards this goal? What if Arthur had returned, as a Gwynedd?