Factual error: When Billy Mack hears he is No. 1, the DJ, (Radio 1's Wes, who does the chart in reality) says the two singles which could be No. 1, then announces Billy is and plays it, without playing the no. 2 song first. The chart is done as an uninterrupted countdown, so even if it's known what no. 1 will be, they'll still play the "losing" song before it.
Revealing mistake: This is really a reverse mistake resulting in NON-visible crew. When the U.S. President's black limo arrives and stops so that the camera is facing the car's left rear side, you can see that the car has been smeared - unevenly - with some sort of gel to prevent us from seeing the reflection of the lights, crew, and the set opposite Downing Street.
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.