if so, then thats classed as entrapment and would be an extremely good defence in court - was there a videotape? if so, and it clearly shows the headlights of the unmarked policecar been flashed to let your mate out and then giving chase then i would think that the evidence is classed as inadmissible.

even if it was a marked vehicle, then it is still classed as entrapment as it is enticing you to speed to overtake, which in itself, speeding to overtake is not illegal providing it is done safely and you return to the highway limit or below (not sure how that ruling works in 30mph zones, but i'm confident its correct for de-resitrcted zones)