ah, it’s a Royal thing; the Royal Mail uses ER at the moment, as it stands for Elizabeth Regina (Regina = Queen), but it changes with the monarch, so when it was queen Victoria it was VR, and when it was king Edward it was ER as well, but that stood for Edwardus Rex (King). If you snoop around the UK you can find old postboxes with different initials on them, which gives you a clue as to when that particular postbox was made!