Dave GKX 478N in your pictures is owned by club members Les & Val and it is a genuine ex Herts Plice motorway car. They also have an ex Hants Police car which is full has full livery and was fitted by Rover with a 5 speed gearbox (experimental). GKX 478N was bought and saved by Pete Capnos (Bristol) many years ago after it had laid derilect for some time and he fully restored it in the late 80's/early 90's. In 98 or 99 it changed hands to Terry Wells (a real police car buff) who then went and found the correct signage and roof light box. Les & Val bought it off him. The car has been on many of our club stands at the NEC over the years and also on a club stand at Earls Court (93 or 94 if memory serves), the last occasion was 2008.
The first time I saw it was in 1994 and I had just suffered brake failure on the way into the Malvern Show at the Three countries showground, Worcestershire. I was under the bonnet and in the process of disconnecting the feed to the back brakes and blanking the feed, when out the corner of my eye I saw it roll past and stop and someone get out. (At that time West Mercia Police were still using orange stripped cars). A voice asked me if I had problems to which I replied 'nothing I couldn't fix' and then looked up and saw the car properly. Pete Capnos then asked me if we were going to the show and said he would let the organiser on our club stand know that I was going to be late because of the breakdown. When I realised that he was not a copper and the car was retired from active service , I did breath a big shy of relieve!
Anyway, most UK forces had P6's at some time or other from 63 to 77 and they were used both in plain and in livery. Devon and Cornwall also had a white Estate which incidentally was recently on ebay but is not in livery. They also had Mexico Brown cars as well. Gt Manchester Police had white, blue and red P6's which were used on motorway work. The red ones tended to be plain apparently.
As for the MET they had white, blue, brown and black P6's according to the production records and I have seen news films where they are present in full livery.
Going back to your picture of GKX, we also have one of the coppers who drove it in service in the club and at the National Rally in 2007 at Turweston, we reunited the car and driver for the first time in many years. GKX's sister car was dramatically written off on the M1 Motorway, when it stationary on the hard shoulder, while aiding a broken down vehicle, when it was run into by a bedford lorry. The lorry ended up nearly on top of the car and GKX went out to assist in the clear up.