A History of Celtic Britain (2011) BBC

Episode 1: Age of Iron

Neil Oliver continues his epic story of how Britain and its people came to be. Diving for 3,000-year-old treasure and pot-holing through an ancient copper mine, he discovers how a golden age of bronze collapsed into social and economic crisis set against a period of sharp climate change and would eventually be replaced by a new era, of iron.

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