Miss Pettigrew Lives For a Day
(2008)

Over the course of a terribly eventful day, a down-on-her-luck governess, Miss Pettigrew, masquerades as the social secretary of Delysia, a flighty rising star with too many boyfriends to handle.

In the end all is put right: Delysia (Amy Adams), choosing love over money, asks Michael to marry her. He says yes and they leave for New York to start a new life together.

Joe (Ciarán Hinds) finds Miss Pettigrew (Frances McDormand) after she is revealed as an impostor and asks her to marry him. She agrees and they walk off to get Miss Pettigrew a long overdue meal.

Thanks Mary C!