The British Isles are a group of islands northwest of Europe. Not all of these islands are British.
The Isle of Man lies in the Irish Sea between the two main islands. The Channel Islands are British Crown Dependencies that are located to the south of Great Britain in the English Channel and are physically closer to France.
The term "British Isles" is used for convenience in grouping the following countries into one category: to inderstand this better see What Is Britain? a presentation by Audrey Collins, Records Specialist at The National Archives.