Caractacus or Caradoc was actually chieftain of the Catuvellauni, a Gaulish tribe that migrated to Britain a few generations before his time. He unsuccessfully fought Claudius' invasion in A.D. 43, lost and fled to the west, living amongst the Silurians. He became leader of the western tribes' resistance, a kind of high king like Vercingetorix, and fought a very successful guerilla campaign against the occupying legions until AD 51, when he was forced to give open battle. His family was captured and he sought refuge with Queen Cartimandua of the Brigantes in the north. She gave him up to the Roman governor. Caradoc and his family were brought to Rome and allowed to live there in exile for the remainder of their days. He wasn't executed in public like Vercingetorix several years before.

More info here.

A great book, highly recommended, which slightly fictionalizes the events of Caradoc's life as well as that of Queen Boudicca (Boadicea) is The Eagle and the Raven by Pauline Gedge. The historical events and details in the book are quite accurate, from what we know. I picked it up in hardcover back in `81 and have read it and re-read it several times since.

Not that this has much to do with the song, but anyway...