it is quite clear, hackney carriages that is 'black' cabs in london and hackney carriage plated vehicles elsewhere have a meter and the rates are set in stone,
'minicabs' are private hire vehicles and although many companies charge similar rates the response by an earlier aber was correct, you can be 'charged' whatever the driver feels like charging,
when the car arrives always confirm how much the fare will be, its no different from any other type of transaction,
so your fare is normally a tenner, i hardly think twelve quid constitutes ripping you off but in order to keep goodwill many cab companies expect all their drivers to operate to a fare scale.