There's a lot of murkiness to the ancient history of Britain, made more complicated by the Saxon and Anglo-Norman invasions. But if you're looking for the foundation myths than there is a very obvious choice in the legend of King Arthur. If you're looking for some sources older (and perhaps less pseudo-French) than Le Mort d'Arthur, you might look at Pseudo-Nennius' "Historia Brittonum" or Geoffrey of Monmouth's "the history of the kings of Britain." The latter was hugely popular in the middle ages and probably played a major role in the canonization of the story. Both sources take Arthur as the king who unifies the British against the Saxons and trace the lineage of the kings of England back to (where else?) Troy.