The BBC's out-dated insistence that all new comedy writers go through the route of writing for any number of their lame topical comedy shows--thus poisoning them, and ensuring that when they finally get their own show, it will be filled with lame topical gags in place of genuine comic invention. Example: I'll bet you anything Lee & Herring went through the BBC poison mill.
BBC POISON MILL
Meaning of BBC POISON MILL in English
Slang English vocab. Английский сленговый словарь. 2012