I literally just came back from a month in Poland, and UK food prices in supermarkets are just as low if not lower. Certainly Aldi prices easily beat Polish supermarket prices on nearly everything, maybe with the exception of baked goods.

Don't get me wrong - M&S and Sainsburys are very expensive places to shop. But I don't see the quality at Morrisons or Asda or yes, even Aldi as being any worse and the prices are very low. Go to France or Germany and try to compare, I can bet that for your average buyer groceries will be cheaper.