Health Act 2006, Part 1, Chapter 1, Paragraph 2 (2) [my italics]:
"Premises are smoke-free if they are used as a place of work-
(a) by more than one person (even if the persons who work there do so at different times, or only intermittently), or
(b) where members of the public might attend for the purpose of seeking or receiving goods or services from the person or persons working there (even if members of the public are not always present)."
As I read the legislation, your husband can smoke in his workshop (as long as he ensures that nobody else enters it).
Chris