And there is not (unlike the medallion system, as I understand it) any specific limit on how many can pass The Knowledge (the test you have to do to become a black cab driver; before that, you can get a taxi license and work for a minicab firm). The reason it's not instant is that it is one of those "requires a substantial investment of time" scenarios.

So I don't think painting "London's black cabs" as a monopoly holds water.