I assume he's talking about black cars, which are licensed car services, not gypsy cabs. Licensed car services generally are more expensive than cabs and require pre-booking; it is not legal for these cars to pick up passengers who just flag them down on the street (although many do). Taxis, on the other hand, can not be pre-booked (although you certainly can agree with a cab driver to come and pick you up at a certain time) and only may pick up passengers who flag them down on the street.