That's it! The body is very similar to TVR bodied RGS-Atalanta, in the front, while the back is a bit more different. I don't know if this car has modified TVR body or an original one: is there someone who can rubber my doubt?
A point for Nicanary.
The history of the RGS Atalanta make is a bit vague on this. The puzzle car was registered in 1957 to a Mr Petty - it was the 5th RGS Atalanta but was a rebuilt car first constructed by Richard Shattocks in 1944 and registered MMX343. The whole rebuild resulted in what was effectively a new car, and it was re-registered SHO664.
Trevor Wilkinson of TVR was fitting RGS Atalanta bodies to his chassis in 1953, but I believe these were alloy, and TVR later built GRP bodies when the Grantura went into production. So I believe that the car you are thinking about is a TVR chassis with a RGS body. It appears that Dick Shattocks built chassis and bodies to order, and kept very vague records.
The information I have all comes from a Coys Auction entry in 2009.