You decide.
This from the Smoke-free (Premises and Enforcement) Regulations
"Definition of �enclosed� and �substantially enclosed� premises:
Premises will be considered to be enclosed if they have a ceiling or roof and, except for doors, windows or passageways, are wholly enclosed, whether on a permanent or temporary basis.
Premises are substantially enclosed if they have a ceiling or roof, but there are permanent openings in the walls which are less than half of the total areas of walls, including other structures which serve the purpose of walls and constitute the perimeter of premises. When determining the area of an opening, no account can be
taken of openings in which doors, windows or other fittings that can be open or shut. This is known as the 50% rule.
A roof includes any fixed or movable structures, such as canvas awnings."