The theater where Hitler was most likely to use chemical weapons was the Eastern Front, and its still somewhat surprising that he didn't. His generals urged him to do so and, unlike using them on the Western Allies, the chances of direct and immediate retaliation in kind were relatively low.
Using them on Britain is a no-win situation and besides, doesn't really fit with how chemical weapons were generally used. They were a tactical weapon meant to force breakthroughs (and their efficacy was hugely dependent on the element of surprise and on weather conditions), their usefulness in strategic bombing was likely to be extraordinarily low against a combatant nation.