Smoke Free Legislation came in to force in 2007, making it illegal to smoke in all enclosed or substantially enclosed public areas and workplaces. Below are some links to relevant guidance:
The Health and Safety Executive provide information for businesses on smoking at work.
If your business provides a smoking area for customers, you must ensure it meets the smoking shelter requirements described in the Smoke-free (Premises and enforcement) Regulations 2006. This means the area must not substantially enclosed (meaning more than 50% of the area is enclosed).
Smoke Free England have provided guidance for businesses on Smoking.
There is currently no legal provision preventing the public from smoking e-cigarettes within food businesses; it is at the Food Business Operator's discretion as to whether they will allow it within their business.
However, staff must not smoke e-cigarettes in places where food is handled or prepared. Good personal hygiene is also important for staff to practice, as there is an increased risk of hand-to-mouth contamination occurring when an e-cigarette is used. E-cigarettes and any associated items (e.g. 'E-liquids') must be stored away from food, so there is no risk of contamination occurring.