Bounderby is stressed about how to tell Mrs. Sparsit that he's going to marry Louisa – good call, since she was totally hoping to marry him herself. He is expecting her to faint or to attack him or something when he tells her. Instead Mrs. Sparsit instantly takes on a superior, condescending, sarcastic tone.
Since a married man doesn't need a housekeeper, Bounderby offers Mrs. Sparsit a position as housekeeper (or something – it's not really ever clear what she does there) at his bank, and she accepts.
Bounderby and Louisa have a six-week courtship period, then get married.
At the wedding breakfast, Bounderby makes a blustering speech about coming from the gutter.
Their honeymoon is mostly a business trip for Bounderby to inspect some of his other factories.
As they are about to leave, Louisa hugs Tom, who remains a selfish jerk and thanks Loo for sacrificing herself for his sake.