There is no specific legal requirement for employers to provide defibrillators in the workplace.
However there are many documents and statements where it is guidelines for certain locations to have an AED. For example the Resuscitation Council and the General Dental Council issues guidelines in 2006 with regard to dental surgeries. http://www.resus.org.uk/pages/MEdental.php
The Health and Safety Executive’s syllabus of first aid training for offshore installations does include the use of defibrillators but this is not extended to onshore first aid. However, the Health and Safety (First-Aid) Regulations 1981 do not prevent an employer from providing defibrillators which could benefit both their employees and the public.
Questions on workplace legislation initially should be directed to the Health and Safety Executive.