Here's an interesting new article that I just found on the web:
http://news.yahoo.com/s/afp/20090625/wl_uk_afp/britainjapanhistorywwii
To the best of my knowledge, Britain never considered using gas against Germany, most likely because the UK was close enough to the Continent for retaliation to take place. I wonder if the reason why the use of chemical weapons against Japan was seriously considered was because there was no chance of retaliation. I also wonder if racial prejudice against Japan measured into the equation to consider using gas.