Left Hand Drive Vehicles?

I think they were both Mercedes, which in those days were not common in the UK. Possibly there was an intention to suggest that these people were in the habit of driving on the continent. There are a few cues in the film that might whisper "International Jet Set", a concept that still had some, rather hackneyed, currency at that time.
The doctor's Rover 3 Litre was evidently right hand drive. It looks like part of the scenario is that he is not "Jet Set", but just an ordinary (hah!) Harley Street specialist. Like the winter snow, which you don't get in Cornwall, I suspect this was at the insistence of the producers or the company, and was soft pedalled by the director.

I agree, and so did the major reviews, that this was a lousy imitation of Bette Davis' s 1939 "Dark Victory." Susan Hayward was always Susan Hayward, not the character she played. Very disappointing.
Puleeze--the early scenes showing left-side steering wheels in autos was just a bad goof. Way too sophisticated an explanation that these were the vehicles of jet-setters of the time....at least they had the sense to show right-side steering wheels in the British autos in the seaside scenes in the last part of the film.