I've noticed over the years that, within a region, Police 'panda car' fleets tend to be made up of one or two standard models, usually from the same manufacturer (in my area it's Vauxhall Corsas and Astras), which makes great sense as I imagine they're purchased on a mass contract similar to driving instructors' cars.

However, the RPU fleets tend to be a seemingly random mix of cars, often including Octavias, Modeos, Insignias, M5s, Imprezas, Lancers, V70s etc (although funnily enough I can't remember ever seeing a Merc in Police livery in recent years). Why are these fleets so pick 'n' mix? Do they allow the officers to choose their own vehicle as it is effectively their 'office'?