The UK Health & Safety Executive workplace directives state:
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.
On our site you should find all you need to fulfill your legal workplace obligations.