Warrior Queen (aka "Boudica"), on ITV (UK)
IMDb; PBS (archived); TV.com; Wikipedia

This aired in 2003 in both the U.K. and the U.S. In the first century, Rome makes a treaty with the Celtic Iceni tribe in Britain, but their presence and demands (exorbitant taxes, sending slaves to Rome, worshipping the emperor and abandoning their own religion) become increasingly intolerable. When told to leave, they of course refuse and become much worse than before. So the Iceni queen Boudica (Alex Kingston) unites the previously warring tribes of Britain under her to fight the invading Romans. They have some great victories, and of course, in the end, a terrible defeat. Of course, we also get to see there are some truly evil Romans (such as emperor Nero), as well as decent Romans, who simply have no choice but to follow orders (such as general Suetonius). Anyway, I thought the movie was pretty good. Of course, after being built up to desire revenge, seeing it taken generally tends to be quite satisfying. Dunno what else to tell you....