I have one, but you probably have to be a police officer to fully appreciate the genius of it...
Black Hackney taxis should have a machine in the partition between the driver and the passengers. This machine should accept payment by cash or card, and payment should be required up front.
The machine should show the rate on it as you travel, and alert the driver if you run out of money, and also give the passenger a choice to press a "stop now" button to stop it (at which the driver could stop to let them alight).
The machine should automatically provide the change.
No money changing hands, payment given up front. Would solve a lot of problems...