The external, and most internal, shots of the theatre on Jersey, where the final of the Magic Shield competition is held, were shot at the Theatre Royal in Nottingham. Many of the theatre's ushers were recruited as extras to play ushers in the film.
The idea of Harry making a Pros and Cons list about his girlfriend and being found out is inspired by an episode of Friends where Ross makes a Pros and Cons list about Rachel and is found out.
When Harry Kane is flying back to London from Jersey the aircraft on the runway is a Flybe Dash 8 Q400 which does fly that route. The interior in the next shot however is filmed in a much larger aircraft with 6 seats across (the Q400 has 4 across) and while he is having his panic attack Harry refers to the oxygen masks in the safety briefing which the Q400 doesn't have.