The British National Anthem represents the whole of the UK.
However, when an anthem is needed for one of the component countries of the UK, for example at an international sporting event an alternate song is used:
Wales - Hen Wlad Fy Nhadau (Land Of My Fathers) by Evan James
Scotland - Flower of Scotland by Roy Williamson or Scotland the Brave, or traditionally Scots Wha Hae;
England - England generally uses God Save the Queen (the British National Anthem), but has used Jerusalem or Land of Hope and Glory by Edward Elgar
Northern Ireland generally uses God Save the Queen at events associated with the British tradition, and the Irish national anthem Amhrán na bhFiann at events associated with the Irish tradition.