I think you are really misunderstanding things. The term "Celtica" is not being used as a term to include the British Isles, Iberia, Gaul, N. Italy, Germany, Pannonia, Serbia, and Anatolia, nor has it EVER been used as such.
The problem is that the greek geographs didn't have a precise idea of what was the end of Celtica. Before the arbitrarian borders made by Caesar, "Celtica" could be as well Pannonians than non-celts of Gaul as Ibero-Aquitans or Ligurians.
It's the main reason for why we use here the word Gaul. Besides, even the name of Gallia Celtica only covers a part of the actual celtic Gaul, while covering non certain gallic peoples or political alliance that didn't concerned Gaul only (such as Heveltii)
The term Celtica means Gaul specifically. It is the more ancient term used by the Greeks, and possibly something similar by the Celtos themselves. At this time, only Gauls referred to themselves as Celtos, not the Britons, not the Belgae, not the Celtiberians- just the Gauls. It is like the Greeks calling themselves Hellenes, and therefore we might call Greece Hellas as another name for the same place.
The problem, again, is that the Greeks called Keltoi everything they found between Atlantic and Mediterranea. If it was in Gaul, they called it Celts.
For the celts outside Gaul that didn't used the word, it's dubious. What is certain, is that greek and romans didn't used Keltoi for Brittons. That said, they used it for continental celts including Belgians ones.
Furthermore, there wasn't a real difference between Pannonians and Gaul, except the political borders, at the contrary of Celtiberians or Lepontics.
Or we're calling Celtica all celtic peoples, in a modern sense; or we use it to call objective Gallic peoples including the ones outside "regular" Gallia (including Danubian ones).
Again, the Gauls called themselves first by their
For your exemple for Greece, it's quite my point : all of greek settlements in Mediteranea were greeks, but because they didn't all belonged to Hellas, we (and they) used different words : Great Greace, Pontic, or Massaliots.
They all acknowleded being greeks, but were first about their more immediate identity.
Besides, i'm quite astonished by your claim Beglians weren't Gauls : there is in the sources many things that would proove the contrary : the facts that ancient authors called Belgians peoples we know being Gauls, the fact the language was similar between southern and northern peoples (not talking about Galatoi). In fact, the only non-Gaul people of Belgia would have been Germans and not part of celtics people.
So, i think there is a misunderstanding here. What you call Celtica, is here mainly for or the "(Gallia) Celtica" that is only a part of the Gaul people while including non-celtics peoples (but having sometimes a celtic domination) as Ligurians, Iberians of Gaul, Aquitanii, or it's used for the whole Celtic world.
For Gaul, we use Gaul or Gallia.
What i meant, is that an united celt kingdom, that would be rid of division among peoples of Gaul, or elsewhere, would have been impossible as a real state, and would have looked like more as an hegemony.