As the title says "4 pints of Milk for only £1 in Iceland".
I noticed that 2 pints was eighty something pence, while 6 pints was over £2 also in Iceland. While a similiar 4 pints in my local M&S was over £1.70.
This seems to be a regular price, and not a promotional deal. I can only guess it is a loss leader to get customers in to the store.