The Fire Precautions Act, The Fire Precautions (Work Place) Regulations and many other pieces of Fire, Safety and Building legislation govern the servicing of Emergency Lighting Systems. Some say it outright and others only imply it. Some legislation carries with it heavy penalties. 

Whether you believe you are obliged or not, you have a lot of capital invested in your Emergency Lighting System. This would be wasted money if it didn't work. When you need the Emergency Lighting System work it will be to save someone's life so it must work properly. The only way you can be confident that it will work is if a professional company using properly trained and qualified engineers regularly services it. BS5266 requires Emergency Lighting Systems to be serviced at least twice per yea.