BRM?
Bingo! Not RGS or HRG as suggested, but, of all things, BRM. I think that's about it - since I know nothing more about the car I reckon all the info has been provided, and Carnut gets the point. Sorry TT after all your work.
The car is pictured at Chateau Impney driven by Jonathan Cobb. It is captioned as a 1954/55 BRM-Jaguar Mk1 (3.4) so clearly an XK engine. It is conceivable that the "M" stood for Microplas I suppose, and maybe the "B" and "R" were the intials of the builder/builders. It's all a bit odd that someone would choose to call their car a BRM at a time when they were a well-known racing marque. I don't believe for one second that this car has any BRM input by way of chassis parts or whatever - the V16 was coming to the end of its career, and the team were running a Maserati 250F as an interim measure whilst the P25 was developed. They certainly wouldn't have assisted or provided parts for this car.
If anyone can provide more info, please feel free to post it.