In the mid to late 70's, the Merseyside Police used to have a couple of them, unmarked and in plain green, and if I remember rightly, often driven by WPC's.
Not sure if they were simply for the Chief/Senior officers to be ferried around in as the few times I saw them around Brownlow Hill/London Road, they seemed to have a couple of what looked like senior officers sitting in them.
I have an old mate who was CID/SB back then, I must ask him if he remembers.
There were also at least a couple of dark blue mk1 RS2000's used by plainclothes. I had a ride in one of those (not the Police ones) back in the early 80's, and for their day, they were a seriously poky car. A nice unstressed engine for that bodyshell.