Factual error: The place where Jamie is writing his novel is revealed to be Marseilles, France, when he flies back to propose. In the first part of their story, it might have been Portugal; his housekeeper spoke French, which is a common second language in that country; and Aurelia only spoke Portuguese. Yet in the entire second part, no-one speaks any French at all; only Portuguese and a few words of English. Sure enough there are many Portuguese families living in France; but they do not have their own linguistic enclave in Marseilles, and any generation growing up there would have learnt French as well. Either the language or the airport name are wrong.
Other mistake: Considering how long the Prime Minister's car is parked outside in front of the only road into the school, where there is to be the school Christmas concert, it's surprising how many cars are still trying to find a parking space. Also, you can see that no cars are driving when Natalie gets out of the Prime Minister's car.
Audio problem: During the wedding scene with the choir reveal, various people enter with musical instruments. When the saxophones are playing that they sound like trumpets.
Continuity mistake: Toward the end of the film when Sam reaches the gate from which Joanna and her mother are departing, he looks down to see them in the boarding line with several people behind them. When he races down to say goodbye, Joanna is last in line.
Character mistake: The Christmas card to the Prime Minister from Natalie misspells Christmas - the "r" is omitted.
Continuity mistake: When David opens his briefcase he finds a stack of cards inside with a sticky note that reads, "Read these - a random sample!", but later when he removes the rubber band and reads the cards, the handwriting on the sticky note has changed. (01:35:50 - 01:41:25)
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.