I think the interior shots are studio mockups. The upholstery of the seating unit backs in the compartments goes higher than any I recall, and in the scene with Wisdom in the corridor the right hand bodyside seems to bounce about noticeably when he bumps it. The scene in the toilet would have required the partitioning to be removed to allow the camera to operate.
Regarding the exterior shot, yes, I think it is one of the post-war extra 2-HALs 2693-99, with the body style of the SUBs but an interior layout, and thus window/door arrangement, of the HALs, which were built theoretically as replacements for pre-war stock destroyed in bombing at various points. The Southern probably got some special compensation for these which accounts for a specific build. Known as "Gatwick" units because later, after the film, they were allocated to the Victoria-Gatwick service which started in 1958 when the airport opened. I was a little thrown because he is leaning out of the toilet compartment window at the rear of the third coach of the formation, on the left hand side, and the toilet window on these units was actually on the right hand side, with the corridor (different window pattern) on the left. Then I realised that of course the whole film is reversed. So I believe it's a 2 x Gatwick 2-HAL formation, both running with Driving Trailer Composites leading.
A driver will hopefully be along soon to identify the stretch of line. At the time of the film these units were operating on the Eastern Section. Possibly in the hills beyond Swanley Junction.