But the supermarkets might implement this or they probably already do. There is a pretty cheap version of the items mentioned (bread, milk and eggs) available in all supermarkets and the profit is probably pretty low and they could well be a loss leader to get people in to also buy more profitable items.
I'm talking about milk if you buy 4 or 6 pints, a sliced white loaf for £1 or less and 6 mixed free range eggs for about £1.50. Also the veg specials.
Smaller shops can't compete and the supermarkets will be hardcore with the suppliers, telling them 'we want to sell this for £1 so we will pay you X for it' leaving the supplier having try and supply it for that price.
Look at all the veg for pennies at Christmas. I saw a quote from a greengrocer who was obviously affected, saying something like he was being put out of businesses so people could save £3 on their Christmas veg.
Even if key items are very cheap, its only a minor contribution towards a household's CoL, so will barely be noticeable, but could have significant impact for suppliers so I agree with you on that point.