Actually when you look at the words, Rule Britannia isn't particularly imperialst - the words aren't about ruling other countries, they're about controlling the sea so as to ensure that "Britons never shall be slaves", in line with the principle laid down in court in the Sixteenth Century (though frequently ignored subsequently) that English law did not recognise slavery on its soil.

People often read subsequent history into National Anthems. People listen to the US Anthem, and hear it as about an overwhelming world power, rather than the relatively weak country struggling desperately to hold on to its independence. In the same way Britain in 1740, when Rule Britannia was written (by a Scot as has been pointed out) wouldn't have felt by any means a dominant country in Europe.

Not that that's got anything to do with a possible English anthem - Rule Brittania would never do.