Just to put my thoughts in here. Tesco pay their dairy farmers amongst the highest prices in the UK for a liquid milk contract. See the league table of prices here:
http://www.dairycodatum.org.uk/calculator/calculator_varying.htm farmers that supply Tesco have to submit their cost data to Tesco, Tesco then work out the average cost of production and this is the minimum price they pay farmers. So the farmers are getting the average cost of production (of the entire pool of farmers).
As far as I know they have no plans to change away from the system so the whole fact that they are now selling milk for £1 for 4 pints won't affect their farmers??? as Tesco will be taking the hit
In fact, as asda, aldi, lidl, iceland etc have been selling 4 pints for a £1 for a while now perhaps Tesco's are trying to increase the amount of milk they sell - this will benefit the farmers supplying them - as more milk going into tesco's rather than other markets means more milk being paid for at the Tesco 'premium' price.
Apart from the terrible adverts using beef cows & being pretty poor at communicating I'm not sure they have done anything wrong?