My reasonings for 1801, as opposed to 1707. The Kingdom of Great Britain & Ireland were each single entities when then merged. The Anglo-Irish Treaty is inclusive as it created the current status of the UK. The Norman invasion is best left 'only' in the England article. Why do I see it this way? If we go back far enough, the UK would be divided into many kingdoms - Strathclyde, Essex, Sussex, etc etc.