As I understand it the Health Act 2006 gives landowners the power to ban smoking on land they own.
Thus hospitals can make smoking illegal in their buildings and grounds and the railways on platforms whether enclosed or not.
Not that act - the Health Act 2006 declared all public places and places of employment smoke-free, provided they are enclosed or substantially enclosed. The list can be altered, but only by an "appropriate national authority". Individual owners, proprietors, and even local authorities are not given any powers to designate places, but they may have some by virtue of other statutes or common law, or perhaps by delegation.
What is "substantially enclosed", and who does the enforcing (local authorities), was in The Smoke-free (Premises and Enforcement) Regulations 2006:
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.
On those grounds Swindon Bus Station isn't covered - but it can be added by one of those appropriate national authorities. They must be defined somewhere, probably another regulation, but I can't find it.
