Today's Daily Telegraph carries a journalist's horror story regarding a violent driver refusing to accept her fare, despite being Vacant...
In response to her predicament, I wouldn't accept the taxi owner/manager's plea of ignorance regarding the unauthorised driver. The person responsible for that taxi knows who it was leased to for that shift.
Thus I would be petitioning the Ministry of Transport regarding this incident.
Otherwise, for scenarios involving one word against another, if possible passengers should discretely employ the recording functions of their mobile phones. Just in case.