Also, as far as I know, Great Britain doesn't actually exist except as part of the official title The United Kingdom of Great Britain and Northern Ireland. Its use is similar to describing the United States of America as 'the States', or 'America', and is to be avoided.
As a part of the title it is as valid as saying "The United Kingdom". Historically it is more accurate. Giraldus Cambrensis used it, as it distinguishes from Less Britain, currently known as Brittany.