So what?

Nearly all minicab drivers in the London area are self-employed.
That they are self-employed does not mean they can accept
work on their own. TfL has mandated that any booking of a
minicab must be done at/with a licensed cab firm. A driver
cannot accept a booking unless he is also licensed as an
operator. (I understand a few are)

Therefore in practice a minicab driver has to be accepted
by a licensed cab firm which is not allowed to take on drivers
if they do not have a valid license.