If you look at the pump handle you'll see a small hole toward the rear and a similar hole at the back of the trigger. By squeezing the trigger fully you can line the holes up, and insert a metal pin or nail and keep the pump going without having to hold the trigger.
One of the reasons the UK doesn't like this idea is that in the States, serious accidents have happened, and not just as reported above. People have gone to pay for their fuel and returned to their car, and, forgetting that the nozzle is still attached to the car, they've driven off.
If the nozzle pulls clear of the car, the back-pressure on the pump is relieved and the petrol comes out at full flow again, all over the court. Or, if the nozzle is stuck, the hose may part - something the filling station owners don't go a bundle on..