You don't have to pull over for an unmarked police car They aren't supposed to pull people over. Certainly if they don't have lights. If it has flashing lights in the car they would drive alongside you where possible and flag you down if you carried on driving normally... in theory then you could see their uniform. If they were in casual dress then you'd have to rely on the ID if you stopped. However you would probably be able to talk yourself out of 'failing to stop for police' in court should you not stop for an unmarked car.
RE: apart from the bit that comes into the built up area where theres a playground Yes I noticed that.