I've not seen it (probably because I hardly ever watch telly) but perhaps it's not a proper 'meaning'? Nationwide are currently showing a series of very short poems which, in general, seek to offer nothing more than a bit of 'self expression' from their authors:
https://www.youtube.com/playlist?list=PLP8zX0EDIZJvHIkahbhp_ELYbBjyqw5F1
I think she was saying that she had enough food (cereal) to last her three years so she need never leave the room but for the fact that the milk was in a different room (the kitchen).
I find it's best not to dwell on the meanings of adverts too much :o)