In the anime Code Geass, the earliest divergence from OTL goes all the way back to between 55 BC and 43 AD, when the Romans attempted to conquer Britannia. When Caesar arrived, one man, who would later be known as Alwin I, managed to unite the Celtic Briton tribes together against the invaders. As a result, Britannia never became a client state and remained a single united entity under Alwin's rule. An entity that would successfully push back any attempts at invasion and remain independent from, if not cultural influence, Roman conquest.
(I always found this idea rather fascinating, but the anime sadly doesn't really explore the real consequences of it. I do wonder what such a timeline would actually be like.)
Seriously though, it would have been interesting. The Britons have tried to form their own independent empire since the Romans walked in, but like the Goths and the Franks centuries later, they couldn't avoid deep cultural influence.
This is just a personal opinion, so don't take it too seriously:
1) Studying History, I came to the conclusion that a deep, ancient and often neglected undercurrent exists in the cultural development of Europe, from the Bronze Age and possibly earlier to today. In practice, two European Cultures exist: the classical Greco-Roman that we study in School on the shores of the Mediterranean Sea, and the more ancient Celto-Norse culture on the Atlantic.
While often snobbishly considered "barbaric", this second culture was actually more advanced than its Classic counterpart, almost "modern" in its approach to issues that we are barely rediscovering today but that have influenced the British (and by extension American) culture for centuries.
2) the main difference is that this "barbaric" culture was honor-based, centered around the heroic figure of a "Defender of the Land": Cuculain in the Irish Myths, Sir Lancelot in the Middle Ages, until we come to the modern concept of the "Super-Hero". Notice that this defender of the land didn't have to be male: think Boudicca, Joan of Arc, Anne Bonny, Lara Croft (all Historic except Lara Croft).
In short, what we have is a gallant, "Knightly" culture, opposed to the "Picaresque" culture of the Med.
3) As far as I know there is only another Knightly culture outside of Europe: that of Japan (ironically). A non-romanized Briton Empire would have looked similar to feudal Japan: while they wouldn't have been Samurai in the strict sense, there would have been a warrior elite following an elaborate code of honor. If that helps, think of the British Islands as a sort of Japan (even with a shogun-like King Arthur) and the Roman Empire as an ersatz China.
4) Also consider that the Gauls, under Roman rule, were very similar to the Britons culturally, but more exposed to Roman Influence. The Battle between the Roman and Briton Empire would have had Gaul as the main battlefield. Think of it as the Hundred-Years War lasting a Thousand Years.
Is that enough for a timeline?