The originator seems to have a good handle on the various boat types. If it was definitely not one of the Short variants, which had some different appearances especially around the nose, then a 4-engined aircraft could possibly have been a Martin Mars, run by the US Navy, still around at the time. Normally they just ran over the Pacific from California, but if a film company wanted a big boat to dress up as Pan Am and fly over to do shots in the UK, that's an outside chance.
Back to the Short boats, and the French Air Force had a small fleet of Sunderlands, painted white, through into the 1960s, used for maritime patrols. Maybe the film company had got their hands on one of those.