As a minimum, a low-risk workplace such as a small office should have a first-aid box and a person appointed to take charge of first-aid arrangements, such as calling the emergency services if necessary. Employers must provide information about first-aid arrangements to their employees.
Workplaces where there are more significant health and safety risks are more likely to need a trained first-aider. A first-aid needs assessment will help employers decide what first aid arrangements are appropriate for their workplace.
Health and Safety Executive Website