"Britain and Ireland" simply won't do in the first paragraph here. The context is "Great Britain is the largest island in this whole bunch of islands off the coast of Northwest Europe". Reducing that group to "Britain and Ireland" makes the sentence nonsensical: "Great Britain is the biggest island in the group consisting of Great Britain and Ireland." Whee. If that was what the author wanted to say, he would have said "Great Britain is bigger than Ireland", which is a true but rather pointless statement.
How to phrase this so it's understood that Great Britain is the biggest of several islands there? -- Paul Drye
Sorry if I was a bit simplistic in replacing "British Isles" by "Britain and Ireland". How about something like "which is the largest island in the archipelago off the west coast of Europe that includes Great Britain, Ireland, the Faroes, the Orkney Islands, and the Isle of Man". --Eob