My friend who is in the MET has told me to be aware that there is a black Astra driving around herts with blue lights behind grille pulling people over then mugging them or car jacking, The only advise he can offer is if you get pulled and you suspect they re not quite right go to petrol fore court more likly to be seen on cctv or get them to follow you to the local station, this can make the real police annoyed but you can only be polite and explain!
sorry its not much help but beware