Iceni and Britain were unimportant for the majority of Roman history except maybe for the fact that Constantine's father was stationed there and/or Caligula & Julius Caesar's campaign there. It was a small insignificant island and the Iceni as a faction are never mentioned in the sources that are mainly cited for this period.
Anyone could easily make a case for Armenia or the Seleucid empire to be in the game instead because they are much, much, much more prosperous, advanced and important civilizations in this time period. The fact that the celts had no written language, very little engineering skill and were insular in outlook makes it a very anglocentric decision to include them over 2 or even 3 eastern factions with cities made of stone and complex bureaucracies. While the Iceni wallowed in their primitive mud and hay huts, rolling in ****.
The Iceni and Britain was a backwards *********** in this time period no matter how many times the British and the anglophiles try to 'big up' these primitive tribes.
They are only in the game for purely jingoistic reasons. Or maybe at a stretch, because that part of the map was missing a faction and for game balance. Still it's a stretch, compare the Iceni to any Major or (Minor!) Eastern faction and it's like comparing caveman to the ancient Egyptians!
The major celtic civilization at this time was in Gaul, fullstop. The scythians and the sarmatians also had much larger impacts on Roman and world history too. I mean one could make a thread : ' List the factions that were more important than the Iceni in this time period'.
I will still buy the game, as I have loved all the TW games since Shogun 1. But you would have to be a fool to not think that Jingoism or copy/paste Averni did not play a role in the decision to include the Iceni.