In Anne Frank: Diary of a Young Girl, her October 9, 1942 entry says that "the English radio says they're being gassed." This is far earlier than I had thought the general public was aware of this.
I refer you to my answer in this thread from earlier in the month which talks mostly about newspapers. Someone else may write more about English knowledge of the Holocaust. See also this section of the subreddit FAQ
/u/commiespaceinvader has previously answered
Why isn't Majdanek considered part of Operation Reinhard? (explaining when the gas chambers were constructed)