The name "kettle" was invented in the late nineteenth century by the drivers of Hackney carriages as an attempt to distinguish between their regular passengers and passengers that used the carriages infrequently. It is thought that credit must go to a character name Arthur Mow.

As you may know the London taxi drivers motto is "He was a stranger, I took him in and made him tea". Tea is made from a "kettle" and thus the phrase "put the kettle on" referring to the stranger became shortened to just "kettle". This distinction helped them to maximise fares for journeys which would not naturally contain a diversion other than that introduced by the cabbie when he had a "kettle" in his cab. It was a subtle way of letting other cabbies know that they were going to take someone "for a ride". So "kettles" are unsuspecting passengers.