doubt they can do you in their own cars as there off duty and to prove it there needs to be 2 coppers if they not got any video or photographic evidence.

i had a similar thing a few years back. did a u-turn legally but as it was a 40mph road i shot off like a bat out of hell (in a 205 diesel) anyway get about 500 yrds up the road and stop at a set of lights. theres this dick in an unmarked car flashing a badge at me. unwind the window to him saying he has me on camera doing 90mph (previously in this car i had to drive about 5 miles in a straight line to get to that speed so someone obviously stuck some nos in whilst i werent looking LOL) and the fact i could not see a camera anywhere in the car made me smile so did the only thing i could really as didnt know much about what could actually happen... "sorry officer wont happen again officer" and he fucked off.

also had a tug for a copper saying i drove past him a bit "loud" so i must have been speeding asking me if i'd "like to lose my licence" it makes you mad that these cunts can tug you when they actually have no idea what speed your doing and you have to just bite your lip and make them feel important.