Sherlock Holmes and the Secret Code AKA Dressed to Kill (1946, Basil Rathbone, Nigel Bruce)

"A close-up of an early LGOC bus - possibly K424 which you show on your site as featuring in a more recent film." [Star!. with Julie Andrews] "Later there is a sequence filmed through the rear window of a car travelling through London.
Another early car follows close behind, and there are some people posed in period costumes to create the atmosphere of the Sherlock Holmes era but the give away that the film was made as recently as 1946 comes from a glimpse of a more modern London bus, possibly an RT."
(thanks Alex)