My understanding on naming of parts is different from that of Annraoi.

It is that "Great Britain" means exactly the same as "Britain", and is a way of distinguishing it from Brittany. (Grand Bretagne and Bretagne.)

Either way it is essentially a geographical expression. Britain - "Britannia" as the Romans called it - is the big island in between France and Ireland, including the little islands around it such as the Isle of Wight and the Isle of Anglesey, and most of the Scottish ones, but not the Orkneys and the Shetlands. (I'm not sure about the Outer Hebrides.)

(I'm having to break this post into bits because the server is playing up again.)