Continuity mistake: When Mia is talking to Harry from near the doorway to his office, just before Sarah turns up for her talk with him, the office chair behind Mia has been wheeled around. There's nobody moving enough in the scene to account for it at that point, though Sarah comes past moments later on her way in to see Harry. (00:17:25)
Continuity mistake: During the scene between John and Judy in the classy hotel set, as they are preparing to stimulate sex against the bedpost, Judy removes all her outer layers of clothing and throws them on the bed. In following shots you can see the arrangement of clothing on the bed has changed. (00:14:00)
Answer: It's because of the complications that Sarah was having with her troubled brother. Apart from that, who's to say they don't eventually get together?
Hamster ★
In the follow-up short film (the Red Nose Day special) we learn she eventually gets her happy ending with a man played by Patrick Dempsey.