From a Wikipedia article on the British Anti-Invasion Plans of World War II:
It seems very likely that the British would have used poison gas against troops on beaches. General Brooke, in an annotation to his published war diaries, stated that he "...had every intention of using sprayed mustard gas on the beaches". Mustard gas was manufactured as well as chlorine, phosgene and Paris Green. Poison gases were stored at key points for use by Bomber Command and in smaller quantities at many more airfields for use against the beaches. Bombers and crop sprayers would spray landing craft and beaches with mustard gas and Paris Green.
General Alan Brooke was in charge of UK Home Forces from July 1940 to December 1941, so his diaries should be pretty authoritative when it comes to the British anti-invasion preparations. Paris Green is a highly toxic chemical that has been used as a pesticide.