I have just found this image of an old advert, and I reckon it's the same car. I had no idea that RGS shells were made by Wemyss Woodhouse, who were the makers of Microplas shells.
I reckon Hermanoto gets the point - the only question left is the date of the work, and I think the chances of finding that are slim. My claim that it was a Bristol 401 was based on something on the Bristol Owners Club site, which mentioned that one such car had been rebodied with an RGS shell (possibly to lighten it for competition?), but I have rechecked the site and the information has disappeared. The OP says it was a 400.