Alfie has received a reply from David Lammy at the Department of Culture, Media and Sport:

As you may know, it is the tune which constitutes the authorised part of the National Anthem in the United Kingdom and not the words. The latter are traditional and the choice of words and verses to be used on any particular occasion is one for those concerned. Whilst the National Anthem is the anthem for the whole of the United Kingdom, the constituent parts of the United Kingdom may quite properly have national songs for which they have a particular attachment.

OK David, so assuming that we have a national song for which we have a particular attachment, how do we go about getting it recognised as the national anthem for England?