Oh, they knew about it alright. I've read quite a few books now about Bomber Command, unfortunately often it seems the priority was shipping maximum bomb loads rather than defence - though of course there was also the issue of how much defence is enough defence and when does it stop being beneficial.
You can debate also using .303 guns against fighters armed with at least 13mm & 20mm if not more.
Or the much maligned Defiant fighter that shot down most enemy aircraft during the Night Blitz....by attacking from below.