If I was in your situation, I think I would refer the matter to the IPCC immediately, and inform the police station, to which you were taken, that you have done that. At least then they can decide if they wish to proceed, if they do, at court you can say that you have taken those actions. It seems to me as if you have been lured into a situation where you could be seen to have broken the law. If my memory serves me right it is called entrapment, and I rather think the police are not allowed to do that.
I think your approach in terms of asking if a fare was requested is valid, if he also said he was lost, that should surely be questionable, I would have thought any police officer worth his salt on duty in the area covered by his own police station should know the area like the back of his hand! Hopefully you can also demonstrate that you do not earn money as a mini-cab driver!
I also agree with EDDIE51 about demanding an apology, and I suggest you take legal advice about suing for wrongful arrest.
I also agree with gingejbee, glad it wasn't me. I also agree that if I was in a city and lost, I'd like to think there would be good samaritan to help me!