The Health and Safety at Work Act (1974) requires your employer to "ensure", so far as is "reasonably practicable", the health and safety at work of all their employees. 
The Management of Health and Safety at Work Regulations (1992) requires an assessment of the health and safety risks to which both employees and non-employees are exposed to whilst at work. 
The Workplace Regulations (1992) requires employers to ensure that arrangements are made to protect non-smokers from discomfort caused by tobacco smoke in rest rooms or rest areas.