Our Celtic Experts will know far better, but if understood what they said then the Casse were actually attempting to unite the british tribes and fought many wars for that. If they would have achieved that they would actually have been a really significant Nation. Also don't forget that they had very very much contact with the Gauls.
Still it is also a matter of preference, as you said, not everyone can be included.
Oh btw. Caesar had a much harder time on his british expidition than when he was in Gaul. And i don't think you would agree at excluding the Aedui to put in Illyrians would you ?