I also found comments on Stephen Fry's site about QI XL from 07/03/09 [1] where bread and milk was apparently discussed, with no mention of Fry going off the handles over the subject he himself apparently introduced. And the comments themselves includes multiple mentions and have been left up.
(EDIT: The episode in question is season 6, episode 11, Films and Fame. It doesn't mention "warm milk" but it does go on quite a bit about bread and milk which GP also claims he has an issue with. Stephen Fry asks what you should not feed a Porcupine; Emma Thompson, who is a close friend of his and certainly would know if this was a no-go area, answers "bread and milk" and Stephen Fry repeats her answer, and David Mitchell and Alan Davies then gets into an extended argument about bread and milk. Here is where it starts [2]. That episode also includes a fantastic segment with discussion about the late Alan Rickmann, btw., where John Sessions does some fantastic impressions of him)
Barring evidence to the contrary, I'm going to assume that if there's a mention of this in his autobiography, that it's meant as a dry joke or exaggeration for comedic effect, which would fit very much into how Fry writes and talks.
[1] http://archive.stephenfry.com/forum/topic/qi-xl-070309-bread...
"He ran a thumb between Clara's lips ... Of honey and sweet warm milk.” “Aniseed?” “If you like aniseed it will..."
There's nothing else relevant except your comment on google or Bing.