Everywhere. Tipically the gov' puts forth a guideline on license use and sells the licenses. In many markets, you cannot be a cab driver without dealing with purchasing a license through or related with the state, and then you have to abide by the state regulations.
Only a few of the guidelines typically contrain options: having to paint all cars the same means that different car quality is masked from the user. Depending on the market, doing a complaint on a cab driver, it goes to the state and it goes nowhere, because the state does not fullfill quality control, etc. In Buenos Aires, the government is also providing an Uber-like app for taxi-cabs.
I dont know the details of London, but most of the time, cabs are what they are because of the government, that has a fiduciary duty to the people they sold the licenses to.
