The Iceni weren't pre-Roman. They were Roman allies in a Roman province.

"Nation" is a tricky term. Most scholars would tell you a nation is a product of nationalism, a thoroughly modern ideology. If you're looking for a Nation-State, then you do need defined borders and all that kind of stuff.

Smith is a British philosopher I've been reading lately who disagrees with the usual interpretation. For Smith, a nation is a self-recognizing group with a shared language, common cultural memories, and a traditional (not necessarily defined) homeland which they may or may not occupy. Smith would see the Jews of the second Temple period as an ancient nation - mutual recognition of each other through cult practice, traditional homeland, etc - while the Greeks, although having a shared language, did not recognize each other as Geek but rather Doric or Attic or by cities of origin. Greeks are what he calls an ethnie, from which in correct political circumstances a nation can arise.

There are some other people who are playing around with the idea of nations. Someone whose name escapes me has made an argument that the Athenian Empire was the result of Athens trying to force the creation of a nation-state. First century Italy is sometimes spoken of loosely as a nation.

For the Iceni, you'd have to draw up your criteria for "nation-ness" and see what the evidence is for the Iceni to fit. Neither Tacitus nor Cassius Dio have much to say about the Iceni - Dio doesn't even use the word. Maybe there are other sources I'm not familiar with that do supply those kinds of details.