Can't see a copper handing over the keys to his car, unmarked or otherwise.
There could be many things illegal about a car that are not immediately obvious, yet I've never been asked for a drive.
My own bike (Ezee front hub kit) will clock 19mph on the stand, not through anything I've done, so maybe 17.5 on the road. I think it's limited by magnetic saturation. However it never actually cuts out, i.e. if I go down hill at 25mph and peddle, the motor still sounds as if it's driving. So, I guess, to the letter of the law, it's illegal.
So I think if the request seemed to come from friendly curiosity I might say yes. If it came with 'litigant clout' I think I'd politely decline pointing out that their body armour would make 'em a bit top heavy and I fear for their safety and that of my bike.