I think we can all agree that any concept of nation and nationality, wherever they are in the world, are social and political constructs. Nations are historically constructed based upon lines on a map, often many hundreds of years old. As an island nation, the lines of the map are rather more easy to define for Britain, but historically, the “kingdom” of Britain could have included much of northern France, and if it were not for the various struggles of medieval Britain, the concept of Britishness, and of much of Europe, could have been very different today.
Nations are also constructed on ethnic grounds. These ethnic divides were accentuated as history progressed, with various ethnic groups settling in different geographic areas, often based upon the same artificial lines described above.
But nationhood is also capable of encompassing ethnically diverse groups. We are quite familiar with concepts of British-Asian, British-Afro-Caribbean etc. and many people self identify as such. National identity along these lines allows different groups to come together and construct a shared identity based upon common values and engaging in common expressions of democracy and government.