Section 2 of the healthy and safety at work act describes the general responsibilities the employer owes their employees, see
http://www.legislation.gov.uk/ukpga/1974/37/section/2
Regulatiom 9 of The Workplace (Health, Safety and Welfare) Regulations outlines the requirement to keep the workplace clean.
Unlike the other posters I think there is certainly a risk to health if live and dead mice are being found throughout the workplace. If poison and traps have been put down for a long time but the problem has not got any better then other forms of control are required. Poisons etc are cheap but don't address the root problem which clearly needs to be looked into e.g. What is attracting the vermin and how are they getting into the building etc.
If you have them, speak to your health and safety rep (if unionised) or Representative of employee safety (if non unionised). Or just speak to the boss about your concerns. You could always mention asking if the local Environmental Health Officer could come and assess the place?
Good luck