Taxi Mini CabMini Taxi Cab
You will find that several Londoners will warn the visitor when booking to take franchised taxi cars and avoiding mini cabins (except those suggested by a trusted company or hotel). Briefly, chartered taxi operators do not have the reputations of misusing, defrauding, robbing or harassing their customers. Riders of taxis in blacks (& those with advertising imprints) have been studying London down to the last detail to successfully complete a professionally conducted test to obtain a license - known as Doing The Knowledge.
The license is constantly reviewed and any complaint made to the London Carriage Office may result in the license being reissued. You will find the driver's license number on a sign in the cabin so that you can "take it with you". There is a significant amount of money that either your own cab or the car is rented by your company, one way or the other.
Beyond London, taxi riders are licensed by the district and can run normal vehicles with a taxi fire on the rooftop & their driver's license is issued in the cabin. You can find them at taxi stands in front of train stops. Minicabins are common vehicles that are used without a driver's license and can usually be rented from a minicab company (Radio Dispatch HQ).
Millions of minicab companies in London - some are mature, well run and reputable - you will find that your London hotelier friend can give you the map and number of a good minicab company to use. Verify that the company is recommending not just a given away map.
Good companies do everything they can to keep good riders busy. ABUT these issues are not warranted - some are incompetent riders, not insured and have a police record. Anyone offering you a mini cab from the curb/street - just never get into one of these cars.