Employees have this right from day one of their employment.
The right can be found in section 44(1) and section 100 of the Employment Rights Act 1996.
The law says that an employee who – in circumstances of serious and imminent danger – withdraws to a place of safety and refuses to return to the dangerous part of the workplace until the danger has been eliminated is protected from dismissal or detriment.
Employees are also protected if they take appropriate steps to protect themselves or any third parties from the danger. What is 'appropriate' is judged by looking at what they knew at the time and the circumstances they found themselves in – and not with the benefit of hindsight.
If you are forced to do this, make sure you communicate your actions and reasons clearly to your employer and make a careful record of what happened as soon as possible, before your memory fades.