The Isle of Sheppey was an island off the coast of Kent.
In the 22nd century, human miners worked on mining acid on the island. An electrical surge resulted in a Ganger being able to act on its own accord. It killed its former controller.
In another mining facility, where a solar tsunami resulted in a group of four Gangers becoming self-aware, Buzzer spoke of the event. (TV: The Rebel Flesh)