Some Welsh dynasties trace their roots to Caratacus (called Caradog Catuvellauni in-game) a Briton chieftan who lead the resistance to the Roman invasion of Britain. He failed, of course, but he held out for a full decade and managed to talk his way out of being executed, which is pretty impressive on his own.

The game also records him as the first King of England, which is inaccurate, but makes taking the country as one of his bloodline feel particularly satisfying. I wanna say it's the Seisylls who have his bloodline in 1066, but you can always check with the ledger.

On another British note, I like playing as the Cerneus. Donyarth (called Dumnarth in-game) of Cornwall was the last king of Cornwall, and his descendant Cadoc in 1066 was the last native Cornish earl of Cornwall. All Breton dynasties tend to have pretty long pedigrees, and trace their roots to Conan Meriadog, a legendary figure who supposedly ruled Dumnonia (modern day Devon and Cornwall) and Brittany, the latter of which he founded.