Mycernius is right about Britain. After both world wars, especially I, when monarchies all over Europe were falling apart (Germany, Russia, Austria-Hungary, etc) strong republican movements erupted, but to no avail.
In Japan, there was quite a call for killing off the imperial line by various shoguns who felt their power slipping, but conservatives persuaded them to exile the current emperor and replace him with a minor. After the war, there were calls from both the SCAP and left-wing Japanese to abolish the monarchy, but thanks to Yoshida Shigeru and MacArthur, they were sustained.