I haven't actually looked in that much on the Celtiberians.
Still the Carthaginian one is viable, as you said they were active in Iberia, but they have held Sicily, Sardinia as well as the Baleric islands. Hannibal invaded Italy add to it.
Romans also had to struggle with Dacians and Burebista I recall, but most importantly, beyond the Rhine river and Germania.
You see they had huge problems with them and was never able to claim those territories.
Plus, Britannia was conquered only up to the land which is "Hadrian's wall".
CastorT's comments