Reports about the Holocaust reached the British and US governments (from the Polish Home Army or Underground) by late 1941, but they didn't quite believe them. They knew about the Holocaust but did not want to know about it.
Reports about the Holocaust appeared in the media in the US and Britain in late 1942 and early 1943 and there was also some discussion about what might be done about it. However, it seems that a lot of this didn't 'register' with the public. In a war, reports about battles are much more interesting than reports about processes.