I pointed out in another thread that this is no such thing as automatic abdication which required both an instrument of abdication and an act of Parliament.

It is unknown, outside of the special circumstances of Edward VIII, when there was no heir of his in any case and he had not been anointed and crowned, for this to occur. It is considered to be a sacred trust bestowed by God, upon the person who becomes, by right and inheritance, the Monarch. So sacred, that even in the case of permanent incapacity, such as that of George III, they remain the Monarch in name and position, although the duties are then discharged by a Regent.

The only nation that I know of where Monarchs do step down as a matter of course is the Netherlands.