It MUST be a home-cooked ham, though - probably a glazed gammon joint - NOT cold, sliced ham. The supermarkets are competing with each other to produce uniquely tasty versions of Christmas hams.
Christmas dinners are steeped in family traditions and it is a feature of every Christmas dinner I've ever eaten that more than one family has shown up. Your mum will bring a different set of traditions to the event from those brought by your dad. "Your" traditions arise from whatever...