Your employer has a legal duty (under Health & Safety legislation) to ensure you have a safe workplace and are not exposed to risk. Therefore they have to carry out a risk assessment on dangers in the wrokplace.
As johnny37 says there should be a H&S rep and there should be a H&S poster displayed giving details of the H&S rep. For more details see
Health & Safety Executive website.
As a result of that risk assessment there should be clear procedures in place to explain what to do in an emergency, what to do when subject to threats or violence and to ensure an appropriate number of staff.
You should use the formal grievance procedure, as Twenty20 says. The grievance would be that you no longer feel safe in the workplace, that there is insufficient staff cover, that there has been no risk assessment carried out and that there are no procedures set out on what to do in such incidents.