The origins of this song are shrouded in mystery. Having evolved something in the late 16th Century, there are versions which are almost never played including verses "To Crush the Scots". As the national anthem for Great Britain, it's place is virtually unique having never been adopted and over a country that is actually made up for 4 others.
When Australia played Great Britain in the Rugby League last night, this song was sung, and I happened to make the comment that within four year that this will probably be replaced with "God Save The King".
In the USA this is perhaps better known as "America" with the first line of "My country, 'tis of thee", and when I saw England play Lichtenstein in a Euro 2004 qualifier, they played England's national anthem "Land of Hope and Glory" followed by "God Save The King" which serves as Lichtenstein's national anthem. Perhaps people in the crowd may have know this, but it was interesting to hear an English crowd singing the national anthem of another country.
In the German film "Titanic" of 1943, the song was used as a propaganda instrument, but in all honesty if you saw this film through the eyes of the English at the time, they would have thought that it was in fact a British film.