I believe it may be part of the armed response unit in sussex and it is indeed a gunmetal evo. There are also two or more silver C class(i think) merc's in sussex, and along with a few dark coloured volvo T5 estates and saloons, the usual omega's and a 406 or two. There are also reports of them using some sort of people carrier as well. However, most of these can be spotted (especially the merc's) because they have an additional aerial on the roof for their radio equipment etc)
The main concern is they have an outstanding number of mercedes', volvos and focus's with a custom high visibilty paint job of silver and illuminous yellow with flashing lights on top. These are particularly hard to spot.
I know the coppers at gatwick use an evo for their armed unit. I'm due to pay the armed unit a visit up there in a few weeks as my father is a mason with two of the officers. I'll let you know what i find out