The short answer to this question is "Everyone!". There was a time when the risk to employees was very low, but that time has sadly passed. It is no longer true even to simply say that only public-facing employees are at risk - although these employees are at the most obvious risk still. With the development of terrorism within the UK and the forms it is taking, nobody is safe and everybody could benefit from Self Defence training and Anti-Terror attack preparation training. Even office workers with no exposure to the public are at risk from intruders into the office and in particular terrorist intrusions and attacks.
If a masked man burst into your office wielding a machette or a gun, would you know what to do? Would your staff?
If there was a bomb explosion in your office building, would you know what to do if you were lucky enough to survive it? Would your staff?
If a member of the public became threatening or violent, would you know what to do? Would your staff?
If a member of the public became verbally abusive, would you know what to do? Would your staff?
Some of these threats might be considered foreseeable and probable, others are much lower probability but remain a possibility and one that should be prepared for in the current security climate. There is "Known Risk" and "Unknown Risk", but there is no such thing as "No Risk". Employers and employees, and indeed all people, would be wise to prepare for both known and unknown risk events. However, if certain events are reasonably foreseeable risks, then an employer might be held responsible in court if sued for not providing training to employees who have faced a foreseeable risk.