Dont be mislead by those who assume general employment law is all that governs the public sector - Civil Service rules on fair and open competition are much tighter. Beyond that the MOD selection system (as you will discover when you undertake the selection interviewing course) is competency-based and does not permit questions derived from personal prejudice. I'm afraid you won't find the ability to smoke or not to smoke within any of the core competencies, nor so far as I am aware (no, I don't have them all off by heart - only most!) within any of the functional competencies. So the question is definitely a non-starter. Now, if you had wished to discriminate against coffee drinkers it might have been an entirely different story ...