On Tue, 26 Apr 2016 07:14:45 -0700 (PDT), Steve Lewis wrote:
You can go to the trouble of finding out the phone number of a taxi
firm in your current locality. Or you can just use the Uber app that
you have already installed on your smartphone.

But that wouldn't help in the situtation being discussed where private
hire requires 24 hours notice and I require a cab in my bit of suburban
North London right now for an emergency so need a black cab.

Although if private hire did require 24 hours notice there might be more
black cabs about serving the short notice requirement.