Just want to say :One reason it is said like that, is that it's the most famous culture in that area, I think. In archeology, the most famous cultures often serve as a natural reference point. They cause a fundamental change, so the eras before and the eras after after fundamentally different.
Scholars talk about pre-Roman Britain. It's not a specific year, it's the entire Iron Age era before the Romans came. There was a certain way of life. When the Romans came, that all changed radically. They were there for 400 years. When they left, it did not go back to how it was before. It had changed too much to do that. That pre-Roman era will never exist again. Nothing will ever be pre-Roman again. Everything that came after, to this day, is influenced by the Roman era, including the existence of many famous British cities started by the Romans. The Roman era marks a very distinct historical division.