The term British Isles is a fairly well accepted Geographic term, although when we're talking about them here we tend to say these islands. To be honest it is not something that comes up in everyday conversation here, because a lot of people don't regard the islands as a single entity and therefore in most contexts don't find a need for a collective name.

The use of the term 'mainland' to describe the island which contains England, Scotland and Wales is misleading in most cases, but it depends on the context - if you're on an offshore island off the coast of Ireland then the Island of Ireland is referred to correctly as the mainland. I have a friend who delights in referring to Britain as the intervening (as opposed to the interfering) island.

Where the term British Isles gets a lot of our hackles up is when someone from the UK comes out with the statement in the company of Irish people that "we're all British aren't we". When challenged on this (which they surely will be by the Irsih person) the usual response will be - I mean we're all from the British Isles. This has long been an irritant to a lot of Irish people and is very prevelant among British sports commentators. It's also noticable that when a sportsperson from Ireland is competing internationally that (at least according to the BBC/ITV commetators) they are British if they win but Irish when they lose.