Firstly, an apology to Simon...don't want to be cluttering your thread with non for sale points.
It's a very good question Stu and Andy.
Firstly, you're indeed right, they are not everday vehicles.
Both Simon and I are members of PVEC, the Police Vehicle Enthusiats Club.
Think of it like a hobby as such, there are many preserved police vehicles, from 1930's vehicles to just out of service vehicles...even down to police bicycles.
I'm collecting parts for a Thames Valley Omega saloon...I like Omega's, live in the Thames Valley and have an interest in police vehicles...so for me it's perfect.
Police forces recgonsie this, and some cars have been donated to PVEC straight out of service, or slightly older vehicles rarely used by them.
The Metropolitan Police still own and operate a 1996 Rover 800 fully liveried, just as when it was in service. They take it to shows etc.
Some PVEC members loan thier vehicles to the relevant forces for publicity days, or open days etc.
A big concern, is 'wannabe cops', that is probably the biggest concern of the police and non club members which is understandable.
If you look on the PVEC website, you'll be able to see the vast array of vehicles on there.