If you're asked to play "God Save the Queen," and you can't find the music right away, fear not. Chances are you do have it, under another name. Try looking for "America" or "My Country 'Tis of Thee."
Your next problem might be to shift it into a singable key. Many books offer it in the key of G, which confronts the singers with a series of D's and that last phrase lunging to an E. No trouble for a trained singer, but as public singing has dwindled in recent decades, so has the comfortable vocal range of most assembled groups. CyberHymnal has a score in F, quite accessible to most singers.
Since the anthem is used by tradition rather than any legal proclamation, I see no problem of protocol with shifting the key. In fact, while the Royal webpage for this anthem has a recording in the key of G, at the Canadian Heritage page you will hear it in the key of B flat.
However, for people who enjoy (or suffer from) absolute pitch, or for those who have this anthem firmly in their muscle memory, even a one-tone drop might be a problem. What do you think? F or G?