Allmost all counties police forces use this livery on their vehicles. Be it local patrol, response vehicles to vans, and Traffic division. All forces have 'unmarked' or 'plain' cars available for covert use.
Ambulance use the same style but with Green/Yellow squares on a (what only can be described as a ) 'bannana' coloured base coat. Fire mostly use Yellow/Red squares on Red base.
The rear reflective livery is designed to help people distinguish the shape of the car, helping them to see the outline better. I believe this is better than bolting 100's of strobes to your car and wonder why people cant see what they are doing whilst driving past!
Oh and to the last question, yes we do still have light bars etc, but don't go too heavy on the lights for the above reason.