The price of £1.49 is so universal that I thought it might be price fixing. But perhaps there are two supply chains. One which was contracted ahead when commodity prices were high and results in that fixed price, and another 'spot' price for milk supplied outside those contract arrangements. The retailers will then need to segment the market to dispose of their 'contract milk' at a high price while attempting to maximise their profits by supplying milk at a lower price in a non-standard format. It only seems to be semi-skimmed, at Lidl it's St Ivel, so they may be using it as a way of disposing of liquid milk after they've used the cream for other products.
It's possible that forward buying may throw up other odd bargains.
Damon.