The British did have a couple of squadrons of bombers ready to use mustard gas on the Germans. These were Bomber Command aircraft and the likely targets would have been German cities. IRRC Peter Cadd9ick Adams tells the story in his work on D Day. Churchill envisaged reserv ing the use of Mustard Gas for 1) Any German landing on Brtain. 2) German use of chemical weapons in Normandy.
In practice Mustard Gas was not as terrible in 1917-1918 as we sometimes think. Persistent blister agent used to be one of the more unpleasant settings for any Cold War era exercise, leading to masking up for a long time, protection against vapour, decontamination and for contaminated and clean medical facilities and not forgetting eating drinking, urinating and defecating drills.....
However, during WW1 chemical weapons were an embuggerance rather than a decisive weapon. Wartime casualties were never decisive and could be minimised with good equipment and discipline.Mustard gas, known as "Yperite" didn't stop either the preliminary bombardment or the launch of the 3rd Battle of Ypres in July 1917. Nor was it decisive in the 1918 German offensives.
While attending a unit NBC Officers' course in 1986 I happened to be reading some memoirs from 1918. This was by a Gunner officer who had been in the Kaiserslacht. He wrote about his unit being drenched with mustard gas and withdrawing through contaminated land, They did not have the fancy detection kit we had in the 1980s, but managed gas drills and gas clear with what we knew as "sniff tests." Later in October 1918 his battery were badly affected by mustard gas with many men blinded. You may have seen the painting "gassed". However, blindness was usually temporary. At that stage of the war they were so exhausted that they were happy to be out of the battle recovering for 48 hours.