The rules in England/Wales say that a shelter must not be 'substantially enclosed' which has been interpreted in most cases as no more than two walls and a roof.
There is also no minimum distance from a building but an urban myth of 5 metres developed because of the pattern of smoke flow in heavily trafficked areas, especially outside entrance doors, where the smoke gets carried back into the building and therefore breaks the law.
On the original point about smoking and eating together outside, if it is a designated smoking area then people who eat there do so at their own risk.
As for vans, if they are company property, or privately owned and the owner is paid an allowance to use for business, then they are classified as a workplace and smoking is against the law. If he's smoking in the van BillyWhiz then tell him to stop and point out why and if he refuses report him. If he gets picked up by the relevant authority, whoever polices this one, then the company faces ahefty fine.