I appreciate that British Isles is a geographical term that is disapproved of by many Irish people (although British and Irish Isles is a rather clumsy term to describe the whole group).

As I understand it the largest island in the Isles is properly called Great Britain (at least as a political unit), although possibly just Britain as a geographic term.

All I can say is my geography teacher (35 or so years ago) took the view that England and Wales were Britain and England, Wales and Scotland were Great Britain.