Too much crack this morning?

Having said that I still think in the real world they'd try to do you for failing to stop.
They're generally ACPO ranks though. It's really unusual for marked cars, or atleast it here is down here.
What's ACPO?
The two round here seem to alternate between a Dog van (but fully stickered up and flashers etc) or a what seems to be a highway car (loads of cones in the back - again fully stickered up with blue ligts etc).
You can ring 999 and ask details as they should have already PNC'd your car before attempting a stop (unless you have caused a road offence)
Association of chief police officers.
Basically each chief officer in the country as a sort of board.
http://en.wikipedia.org/wiki/Securitas_depot_robbery
I'm not stopping for an unmarked police car. I'd either carry on at a normal speed until uniform in a marked car turn up, or drive to the nearest police station. I know that once inside the "unmarked police car" you could demand to see their warrant card... but by then you've already got a gun pointed at you so it's a bit late.
A marked police car on the other hand, I can't remember any news stories of them being falsely used, so I'd always stop for one.

...at which point you'll be done for driving whilst using a mobile phone.
(5) A person does not contravene a provision of this regulation if, at the time of the alleged contravention—
(a)he is using the telephone or other device to call the police, fire, ambulance or other emergency service on 112 or 999;
(b)he is acting in response to a genuine emergency; and
(c)it is unsafe or impracticable for him to cease driving in order to make the call (or, in the case of an alleged contravention of paragraph (3)(b), for the provisional licence holder to cease driving while the call was being made).
...at which point you'll be done for driving whilst using a mobile phone.