Well, technically, it is a P6B.
Having said that I don't recall them being referred to by P5, P5B, P6, P6B etc when they were current, certainly not in conversation or at dealers. In the factory, possibly, I've no idea. In the motoring press, yes, sometimes.
To most people they were Rover 3-litre, Rover 3-and-a-half litre (less often I heard 3-point-five litre) or Rover 2000 and 'thirty-five hundred' or 'three thousand five hundred'. I always thought that 'three thousand five' seemed a bit daft and I don't really recall its use. Strangely, the 3500S I recall as often referred to as the 35S. Loads of people used to get confused between the 3.5 litre and 3500 and I wonder if that's why clubs/press etc started to use that terminology.
It's similar to modern BMWs, most 'enthusiasts seem to refer to E36, E39, F10 but how many people know which is which? Mind you, I saw a proper trainspotter yesterday, complete with notebook and camera