Despite being one of Britain's most well-loved films, this was apparently hell to make. Director Henry Cornelius was vetoed on most of his first choices--Dirk Bogarde turned him down, as did Claire Bloom--and he was forced to make it at studios he didn't want to work at. Cornelius' displeasure was acutely felt by cast and crew, as he didn't hide how unhappy he was. He was also apparently quite lecherous--both Dinah Sheridan and Kay Kendall carried whistles on them at all times in case they ever found themselves alone with him. Olive Dodds, Rank's head of contract artists, later testified that every leading cast member came to her at one point and said they wanted off the film.
The film ran into censorship problems in the US, partly because of the implication of weekends of illicit sex and because of the moment when Wendy asks for a coin so she can "spend a penny". References to toilets were specifically taboo in US films at that time.
The London-Brighton Rally is held in November; it would therefore have been pitch dark long before the McKims got to Brighton.
The trumpet playing of Kay Kendall was actually done by Kenny Baker, who was Britain's top trumpet player at the time.
When Alan and Wendy break down, spill the coffee and provide entertainment for a newsreel camera, they are passed by a friendly couple in a 1904 Oldsmobile roadster, registration A 8829. According to a stock photograph from 1966, Dinah Sheridan drove this very car during her first real life participation in the London to Brighton rally, thirteen years after the film was made.