Can my employer stop me smoking outside the premises?

If they own the premises, yes, they can ask people not to smoke or stand on their property. However, it would be more sensible if they allocated an outside area where people can smoke. If people are smoking outside, but on the employers premises, then bins or ashtrays should be provided. If people are congregating on a public pavement to smoke, they also have a legal duty not to litter so must dispose of their cigarette ends and matches responsibly.