I heard a slightly different side of this story. I think it was John Britten who claimed that he took a +8 to Windsor Castle for the then Prince to try out. Although it was reported that he enjoyed driving it , and liked the car , he had already acquired his Aston Martin DB4 (or was it a 5?) and thought that one British sports car was sufficient.
Either way the story is intriguing, and who knows what would have come to pass if he had made a purchase.
I wonder if anyone who was present at that demonstration (from MMC/John Britten?) is still around to verify the account.