1 pint costs £1.
2 pints cost £1.49
4 pints cost £1.95
As a rough estimate, then P pints of milk costs £(1+0.5*log2(P)).
If you want 3 pints, it's cheaper to buy 4. I don't know what the economics behind this is.
Looking into the farmers' costs, a typical UK dairy farm produces maybe 5,500 pints of milk a day, which if this formula appplied would bring in a mere £7.21. Fortunately, the formula doesn't apply: the actual amount farmers receive per pint fluctuates, but is about £0.20; they therefore receive around £1,100 for 5,500 pints. A supermarket selling 5,500 pints at £2 for 4 pints would take in £2,750, but there's transportation costs and the like so that doesn't mean they're making £1,650 more than the farmers. They do profit rather more for single pints, though.
I surreptitiously took these photos as evidence of the Co-op's pricing. That, not my poor camera skills, explains why they look so bad.
My poor camera skills would have made them look bad anyway, of course.