When you clock in, you are deemed on the premises and when you clock out you are deemed no longer on the premises. Therefor, in an emergency clock cards are checked, if you have clocked out and are stuck in the lift / dieing on the bog etc., as far as the company and emergency services are concerned you are not there.
Depends on how they are paid. If by clocking on early, they are deemed to be on overtime, is this going to be more than than what the 'lift allowance' is? Also, unions would take a dim view, on what would be termed 'enforced overtime'; which is illegal.