If you have time to shop selectively you get to know which shops are good prices for selected items.
I go to Lidl about once a week or fortnight for a few things that are better value than Sainsbury's and to see if there are any household or clothing offers - my sister buys electric kettles there as they have a three-year guarantee and I got some good quality liner socks on offer recently.
Apples, chocolate bars, unsalted nuts such as almonds and brazils are good value and good quality. It's 40p cheaper than Sainsbury's for a pack of 6 apples - Granny Smiths - and Aldi's are usually around the same price as Lidl but occasionally may be 10p dearer. Lidl pitta bread - I buy the wholemeal but they also do white - is the same price as Sainsbury's own brand - at 50p.
A bottle of bleach in Lidl is 60p compared with 70p in Sainsbury's. That's for the smaller size 750cl. The bigger bottle (2l) is probably even better value. Aldi's is dearer than either.
Cheese - slabs of cheddar is more expensive in Lidl and Aldi than Sainsbury's own brand cheddar which is the best value of any supermarket that I know of. About £1 cheaper per kilo.
White vinegar is also a great price in Sainsbury's at 40p. They don't usually have the white in Aldi but on the rare occasion they do, it's maybe 5p cheaper. Don't know about Lidl.
Vegetables and salads, such as broccoli, tomatoes, cucumber, celery, are usually (but not all always) cheaper in Aldi than Sainsbury's and you have to check the quality. The same applies to lemons, garlic, avocado.