The defenders are definitely going to use gas if there is an invasion. Back on June 15th, Sir John Dill made the argument that poison gas should be used on German forces that succeeded in getting ashore that could not immediately be turned back. Though there were some objections, on June 30th Churchill received the agreement of the Cabinet on the use of gas for that purpose.
The Germans were still trying to follow the rules of war. The British suffered no such constraints. I didn't comment a week or so ago on how the British were shooting down aircraft painted white and marked with red crosses which were being used in air-sea rescue operations.