Great points. Given the more reliable track record (and accuracy) of recent Western weaponry, Putin probably doesn't want to get into a tit-for-tat with even tactical nukes. This also would more or less open the door to a NATO war, which he's not presently in position to win. As far as 1940 England was concerned, I believe Hitler was hoping for an easy conquest, then British alliance (it surprises me how well this worked with France, who was more complicit with the Nazis than I'd previously realized). Using gas on British would have (as you said) laid Germans open to all-out retaliation (Dresden bombing bad enough, but it could've been worse).