In informal|lang=en terms the difference between grandmammy and grandmummy
is that grandmammy is (informal) grandmother while grandmummy is (informal) one's grandmother.
As a noun grandmammy
is (informal) grandmother.
As a proper noun grandmummy is
(informal) one's grandmother.
grandmammy
English
Noun
(grandmammies)
(informal) grandmother
grandmummy
English
Noun
(grandmummies)
(UK, informal) grandmother