A History of Celtic Britain (2011) BBC

Episode 2: Age of Warriors

Neil and Barry Cunliffe look at spearheads and skulls at Danebury Hill Fort. They discuss a time where numerous hill forts began to expand in close proximity to each other and the finite size of the United Kingdom began to tell. This led to gruesome and violent tribal battles.

In four chapters, largely based on and illustrated with archaeological finds and sites, Neil Oliver explains how, as far as is known, the Iron Age Celtic tribes known as the Ancient Britains evolved and entered European civilization. Their internecine tribal phase was warlike and partitioned. Overseas contacts, especially metal trade, brought wealth and progress. Ultimately, it attracted the superior Roman empire, which would conquer and pacify Britain into a province, like Gaul shortly before, but Caesar's invasion wasn't the definitive annexation yet, that was left to emperor Claudius; even afterward some Celtic traits and even rebellions remained.
Source: imdb.com