This is JGY 280 the First, a 1961-model MkI 3-litre that was pictured at the Rover factory.
155 HYU was a black car and is preserved in the museum at Sandringham, where I took this picture in 1999.
I presume this is JGY 280 the Second, a Mk II 3-litre. Once again, I have no date for it...