This shopping list nonsense has (unwittingly, I assume) exposed what a farce the 20£ shops are.
Most people make a list with the starting point of what do I need/want to eat. Jack makes hers with the starting point of I must spend exactly 20£ and works from there.
Realistically, if you had every foodstuff known to man stashed away in your massive kitchen - and you were actively trying to save - you'd only buy a handful of perishables, while slowly working your way through the pasta factory in the pantry and the aquarium in the freezer. You would spend significantly LESS than 20£, because it's not necessary.
Jack has 3x fish fingers in stock. She bought fish fingers. She has baked beans in stock. She bought baked beans. She has nuts, frozen bacon, instant mash, (sad) lemons, celery...all of which she bought. She's buying tit she ALREADY has in stock, creating duplicates, because she's not trying to save money, she's trying to hit a spending target.