I agree with others about tinned tomatoes, there is a noticeable difference between cheap and expensive tins. I especially like tinned cherry tomatoes but at £1+ per tin I appreciate not everyone will be willing to pay that much.
I shop mostly at Tesco (online), and am slowly working my way through their cheapest ranges, buying one tin/packet/whatever at a time. I always buy Stockwell baked beans, often add a little chutney or fruity sauce depending on what I’m eating with the beans. I dislike their cheapest pastas, tasted very glutinous to me when cooked.
And agree with Greyduster about Roses lime juice, I’ve tried various other brands and disliked them all.
I buy John West tuna in spring water from FarmFoods, two packs of six tins now cost £8, was £7 until very recently. I don’t know how this compares to the price of similar in M&S but it’s cheaper than Tesco own brand.