Churchill (as Air Minister, and then Sec of State for the Colonies) wanted the RAF to use gas, but RAF opinion was rather less keen. The files - from memory in the AIR 9 category at Kew - suggest that there were a fair few 'wrong type of weather'/'wrong type of terrain for gas to be effective' missives sent to Churchill. There was discussion of the use of lachrymatory gases (in the same file), but the efficacy of this was called into question as well. To date, with the exception of the personal recollections of a chap who served in Iraq in the 1920s/30s claiming that it was employed, there is no hard evidence that poison gas was employed; even if the files were weeded, there is more than enough evidence to suggest that gas wasn't a weapon the RAF was particularly eager to use in 'Col Pol' scenarios.