In the UK, smoking is banned in public places. As an employer, you must take reasonable measures to ensure everyone is aware of the law and ensure they don't smoke on your premises.
How to reduce risks
It's recommended that you:
Smokers can no longer smoke in designated rooms inside your premises (although there are some specific exceptions). Smokers must smoke outdoors, away from doors, pedestrian routes and windows that can be opened. Smokers must also smoke away from areas where there may be ventilation intakes and any other places where there might be a possibility of smoke being drawn into the building or drifting into pedestrian accesses.