What was Britain called - before it was Britain

Albion
Albion (Ancient Greek: Ἀλβίων) is the oldest known name of the island of Great Britain.

Today, it is still sometimes used poetically to refer to the island.

The name for Scotland in the Celtic languages is related to Albion: Alba in Scottish Gaelic, Alba (genitive Alban, dative Albain) in Irish, Nalbin in Manx and Alban in Welsh, Cornish and Breton.

These names were later Latinised as Albania and Anglicised as Albany, which were once alternative names for Scotland.

https://en.wikipedia.org/wiki/Albion