Well until recently, my OH was a self-employed private hire driver. Because of the local authority regulations, he was unable to take his own bookings, only bookings through an operator. The tariffs are also set by the local authority, and are obviously set by mileage, not time. Often, he would wait for up to an hour to be allocated a 'fare' and then get paid the princely sum of £1.90 for a journey of a few hundred yards (after loading shopping bags, luggage etc) before being returned to the back of the queue for his next fare. So out of courtesy, I would say yes - tip your cabbie!