As Ethel states, the owner of the land which you're on is free to set their own rules.
However your question seems to relate to the specific legislation which would, for example, still apply even if the doorway opened straight onto the public street.
The legislation only prohibits smoking in 'enclosed' and 'substantially enclosed' premises. Those terms are legally defined by the Smoke-Free (Premises and Enforcement) Regulations 2006:
http://www.opsi.gov.uk/si/si2006/20063368.htm
If you look at those regulations, you'll see that premises can only be 'enclosed' or 'partially enclosed' if they're covered by a ceiling or roof (including temporary structures, such as a canvas awning). If there's nothing over your head, the law does not prohibit smoking. If there is something over your head, you need to consider the area of the openings in the walls (or where a wall could be). If the 'open' part is less than one half of the total area of of the 'walls', it's illegal to smoke there.
Chris