Is anyone aware of a minimum distance that a smoking area should be from the premises entrance door?

You could also update your policy to say how far or even if you are going to allow smoking during work time. You can put the distance at any distanc and don't think you have to provide a shelter because you don't, you cpuld draw a circle on the floor and tell them only in that area. Provide an ash tray of you allow it because apparently littering doesn't aplly to smokers they just through the stubs away anywhere.

If it is one of the few remaining public houses in the UK typically less than one metre including those that make special provision for smokers. As you have not identified why you are asking it is a bit difficult to answer. E.g. company with no public access can enforce exclusion to their site perimeter. Restaraunt fronting straight on to the public highway cannot set limits as it doesn't own the land.