Alex Kingston is indeed excellent in this role. A made-for TV production lacks money for a more experienced cast and elaborate sets, but that being said, they do succeed in painting a portrait of this brave warrior, and take the veiwer into the Romano-Celtic world of the early Dark Ages.
I am a little perturbed by the quibbling over language. We shouldn't be concerned about the script containing modern expressions. . . to be so is to hold the script to artificial and inaccurate standards. In fact, these people were all speaking pre-modern forms of Celtic languages and Latin. (No one spoke English, and if they had, we would not understand a word of it now.) So what, do we demand they all speak Iceni? Latin? Anglo-Saxon? The point is communicating the story, which the film does quite well.
I was a bit bothered by the lack of refinement everyone displayed. Boudicca and her daughters were wealthy, somewhat educated people. I realize one doesn't stop to bathe in the middle of a war, necessarily, but I doubt their behavior was always this rough. The idea that all people before Christianity, who weren't Romans, were crudely mannered and covered with dirt perpetually, is misleading and disrespectful.
But the film has so many plusses, in spite of a few little minusss, that it's well worth one's time.