Here's a loaf from Tesco for fifty pence (seventy-seven US cents). Looks like twenty or so slices, so call it 2.5 pence per slice; about four cents a slice.

http://www.tesco.com/groceries/Product/Details/?id=262358089

As I recall, however, you have to be quick. The cheapest bread tends to vanish from the shelves within minutes of being put out.