I am, in fact, using a UK keyboard and US isn’t even installed.
I discovered this when I tried to do a MIDI Export. It’s KC is set up as Ctrl-Alt-#, and this is what’s reported in the menu. But it doesn’t work - nothing happens!
I went into the KC dialog, where the command also shows as advertised, but when I tried to recreate it, the ‘Type in Key:’ box shows Ctrl-Alt-\ (using either separate modifiers or Alt Gr).
If just type #, though, I get #. As I do with any modifier key combination except:
Ctrl-Alt-# comes out as Ctrl-Alt-
Ctrl-Alt-Shift-# comes out as Ctrl-Alt-Shift-| (a shifted )
So it only thinks it’s American under very specific circumstances.