I was going to answer - probably but to be on the safe side avoid smoking at all...
however i decided to research the matter to get the facts and found that the act (law) stipulates exactly what is an 'enclosed' place:
Enclosed and substantially enclosed premises
2. —(1) For the purposes of section 2 of the Act, premises are enclosed if they—
(a) have a ceiling or roof; and
(b) except for doors, windows and passageways, are wholly enclosed either permanently or temporarily.
(2) For the purposes of section 2 of the Act, premises are substantially enclosed if they have a ceiling or roof but there is—
(a) an opening in the walls; or
(b) an aggregate area of openings in the walls,
which is less than half of the area of the walls, including other structures that serve the purpose of walls and constitute the perimeter of the premises.
(3) In determining the area of an opening or an aggregate area of openings for the purposes of paragraph (2), no account is to be taken of openings in which there are doors, windows or other fittings that can be opened or shut.
(4) In this regulation "roof" includes any fixed or moveable structure or device which is capable of covering all or part of the premises as a roof, including, for example, a canvas awning.