Tesco aren't jacking up the prices, they are reducing them to keep up with the German discounters that came in a drove all the prices down. This is what actually screwed all the farmers in the UK
If Tesco have to match the price of milk at Aldi and Lidl, then they have to buy it for the same price as Aldi and Lidl, as well as paying their 300,000 employee's.
People rejoice in the low cost shopping options of the German retailers, but in reality their model is only sustainable by having unscrupulous methods of practice. Which includes shafting their suppliers and staff.
If British retailers ignored this threat, they would go out of business pretty quick. In the last 10 years we have seen the start of this race to the bottom in our supermarkets. Where we used to have fish counters, bakers, butchers and deli counters. This is all being ripped out and replaced with imitation Aldi shite. In the next decade, supermarkets will only provide a very basic selection of produce, stacked in cardboard boxes on the floor with no staff and automated tills.
Take these discounters out of the equation, and you would see a much more level playing field across the entire industry, especially farming.