As the only forum except for ScimitarWeb that came out well in the research on this picture, here is an update for you. Essentially have some more stuff to add when I have time but most of the research is on web here. Main thing to add is the 3D images against picture. Will more than likely do over this long weekend.
Spoiler alert. Have I found the builder or car? No. Could it have gullwing doors or a flip front as opposed to what I have created? Always possible. Is the car a Sunbeam Alpine size? Yes. Is it Sunbeam Alpine based, even if just a unique underpinning and a body based on an Alpine? Probably. In fact I believe it is, but without the car cannot go definitive.
Essentially the size of the car was resolved by 3D laser scanning the area of Oxford Street and Vere Street shown in the photograph. All the vehicles were created in 3D and placed in image along with the building and road scans. The Thames 300E van, white in lower left corner, was then placed in the overall 3D model and it was all matched up. The car is Alpine size as the wheelbases of the blue car and the Thames could be compared in a way they could not be beofre as the Thames front was not in the image.
Along the way, worked out the road layout at the time, found that most forums are not great at research and generally had a geek fest. Learnt some new design skills and found much obscure but fun info along the way.