Hey, It's That Voice!: You probably wouldn't realise it unless you've got a very good ear for music, but one of the singers of the England World Cup 2002 anthem in the film is Lily Allen, who stepped in to cover up for Atomic Kitten member Natasha Hamilton after she pulled out. Lily didn't actually appear in the film (though her father Keith did, as one of the in-universe performers of the song), hence why most people don't realise she was involved.
The Other Darrin: Bradley Walsh and Dean Lennox Kelly played Doddsy and Tonka in the film; they were replaced by Steve Edge and Renton Skinner respectively for the TV series. If Interim Manager gets made then Walsh will return to the role, reversing this trope.
What Could Have Been: The film was originally supposed to end with Mike quitting the England job despite his eventual success, and moving to Bolivia to coach there. The producers decided they wanted a Sequel Hook however, and refilmed the ending to have Mike proclaiming "four more years!" (In the TV series we find out that he lasted just over one year, getting sacked after losing every single match in the Euro 2004 qualifying tournament)