Evac chair training isn’t mandatory/a legal requirement (yet), but as a business, ensuring that mobility impaired individuals are provided a safe evacuation route in the event of a fire is required under the Disability Discrimination Act (DDA) 1995 and the Regulatory Reform (Fire Safety) Order 2005.
So, if you do choose to have evacuation chairs, then you must ensure you provide adequate training to any employee who would be the responsible person for using it in an emergency. In any case, your business must provide a way for people with disabilities to exit the premises in the event of an emergency.