"My understanding is the Roman's used the term "Britannia" on its own for Great Britain. Ireland was Hibernia."

Originally the term, or variants of it, was used to describe all of the islands including Ireland. That predates the Roman conquest by a long way. The Romans initially called Ireland by the name Hibernia and GB was named Albion. I think you are right that they would be the first to later specifically call Albion by the name Britain. Probably due to the fact that the bulk of the island (though not all of it) formed the Roman provinces of Britannia. For most of the Roman period Britannia was divided into two provinces. Britannia Superior was centred on London and Britannia Inferior was cented on York