Answer: "Love is All Around" by The Troggs
The film opens with Billy Mack in the recording studio, creating his new single, a rip-off of The Troggs' hit song, with a version he calls "Christmas is All Around". The movie tracks Billy's progress with the single, appearing on radio interviews, television music shows, and elsewhere as he tries to promote it all the way to the No. 1 spot (past the single in the running by boy band, Blue, whose poster he defaces on live TV). Somehow, Billy always manages to make things difficult for his manager, but in the end the two of them come to the realization that their platonic love for one another is what gets them through it. While Billy and his manager have no clear connection with any of the other characters in the film, his song and TV appearances can be heard and seen in the background of the movie.