Well the song was written in 1745 and was adopted as the National Anthem at the beginning of the 19 Century. God then stepped in to save us being too nice to our colonial friends and the undeserving poor of the planet. So, having been saved, we retained the song out of our natural predilection for nostalgia and history. History continues but nostalgia isn't what it used to be.
The Sex Pistols revived the song's fortunes in 1977 and today we only sing it when Liz is around - which isn't very often.
The habit of standing in theatres and cinemas came about because of the impossibly hard seats in those places and to enamble the less ruly to flick lighted cigarette ends on to the waiting bald pates of those in the expensive seats.
We play it before international sports events simply to annoy any americans that might have accidentally appeared thinking Wembley was a theme park or a burger bar.
A lot of people would like a new theme song and a Mr. Billy Connolly, of whom you might have heard - though not understood - suggested the theme from The Archers, a good rollicking number and splendid for carrying a flag to.
(Yes, I am taking the piss, just in case it was over your head.)