You do not and should not stop for unmarked police cars, even with BLUE LIGHTS, you are NOT breaking the law in not stopping either, they have to get a fully marked police vehicle to do the stop, this is because people fear stopping for dodgy people pretending to be cops.
I came across a lady on her phone as we both stopped alongside each other at some lights, so I shouted "either put that phone down or I'll nick you", she looked embarrased and put the phone down, had me 11 year old with me too,!
tabetha