Not 40s, but some proper old London accents in this clip: https://twitter.com/BBCArchive/status/1359460604252192769?s=20 Contrasting nicely with the interviewers final RP 'Thank you'.
For a real idea of how men spoke the Calling Blighty series of films is a treasure. Check the posts on this site for links .Because they filmed men from all round the country you get a good spread of regional variation.
I feel like this officer realises a few seconds in this is for public consumption, and RPs things up a touch.
Fantastic bit of broadcasting RP invective shared by 38 (Irish) Brigade on Twatter. "This slinking Fascist" 'Desario Birancati, Vice-Prefect of Catania, undergoes interrogation in the courtyard of a former Fascist HQ in Catania.'