British isles is a geographical term that describes the islands that form part of Brittania, remembering that much of Britain was part of NW France or Brittany today. The Great (or Gran) simple means big as a way to describe the largest of the two major islands. However history is also important and for many the word British has deep seated political connotations even if the geographical explanation is technically correct. No idea what to call it but the Republic of Ireland, Northern Ireland, England, Scotland and Wales would seem a bit on the long side.