The Rover 2000 (P6) is a saloon car heralding from the 1964-1976. It certainly has a loyal following, even today, and serves as alternative retro car to the Interceptor in the game, being found in exactly the same areas as the Jensen. It isn't as fast, and it's acceleration is worse.
The model feautured is known as the 'B' model due to being the facelifted version. This can be noted by the 'Honeycomb' style grill, the black vinyl on the 'C' pillar and the raised bulge on the hood to accomodate the Rover 3500 V8 carburetors (However, all 'B' models, regardless of being 2000, 2200 or V8 have these bulges).
This car is very commonly associated with both the Police and Criminals in the 1960's-70's and in popular culture can often be seen in British gangster movies and old TV shows such as 'The Sweeney' and 'The Professionals'.
(Thanks to Razo for the name).