OK, it was over 70 years ago, but in WW2 we were, it seems, quite happy to consider the use of chemical weapons against the enemy then, the people of Nazi Germany. See http://en.wikipedia.org/wiki/Operation_Vegetarian. In the event, these were not used, but clearly we had thought hard enough about using them to test them on a (not that) remote Scottish island.
It annoys me when the UK and USA behave so "high and mighty" against the use of chemical warfare when, even very recently, both nations actually used them in Iraq, Afghanistan and of course by the USA in Vietnam (Agent Orange). Perhaps we learned that their use was a bad thing?