Taxi drivers are licenced, so I assume they must provide proof of hire & reward insurance.
I am thinking what I had to do near 50 years back as a London mini cab driver, not a taxi, we were called "private car hire operators", all you needed was a car that ran, they gave you a 2 way radio to communicate with the office, then away you go, but you had to show hire & reward insurance for the car for passenger transportation protection.
Wasn't cheap, cost me then about 1000 quid a year.....????