I would agree it's definitely a Rolls-Royce (or Bentley) prototype of some sort, somewhat badly disguised. It looks a bit on the smallish side to be a Silver Shadow prototype but I guess it could be. More likely it's either the Bentley Burma or Rolls-Royce Tibet project . . . and I tend to think it's the Bentley. As to a date, since the Silver Shadow bowed in 1965 I would date it to just prior to that . . . 1963-64?
Added note: Found the very photo after a brief search - this is test driver John Gaskell taking a break during tests at LeMans with a Burma test car.