Like many of the others have said, definitely Celtic elite/noble unit with more of a continental feel, I think. I would hope that they would be part of the Aedui faction, (personal favorite), but, since the Celts were well known as mercenaries, I could see these guys hiring themselves out to the highest bidder.

All that aside, my gut feeling after seeing the unit was that it is an elite unit of the Nervii, but Gertgregoor said no Belgae...

So, just a guess... elite infantry of the Aquitani?