What do you think of this concept art as a replacement for Civ5's kilt'n'woad Boudicymraeg? I think it's about as good as we're likely to get.
Her garb is perhaps not as Romanized as one might expect from a family of collaborators, but certain excuses could be made for it. Perhaps she's eschewed Roman garments as symbols of the hated interlopers, or perhaps her Roman gowns were simply unsuitable for roughing it out in the wilds, and only a traditional woolen cloak would do.
And of course the developers want to meet gamers' expectations of what a Celt should look like.

So... a happy medium, perhaps?