Aldi: Most things. Eggs, bacon, sausages, steak, belly pork, chicken, their version of peperami, salmon fillets, olive oil, pesto sauce, cheese (huge variety of both cheap and posh/specialist these days compared to a year or so ago), soft cheese, butter, milk, double cream, and all veg. Higher-carb stuff for use in small amounts - humus (various flavours), prawn cocktail, falafels. Nuts - unsalted and salted pistachios, unsalted almonds, pecans, walnuts, hazelnuts etc. They also do ground almonds to use as flour. Also two 'treats' from there: their honey roast peanuts are the lowest carb I've ever seen and very cheap, plus their expensive dark chocolate, peanut & sea-salt snack bars (in packs of 4) are the lowest carb things of that nature I've ever seen without paying crazy money.
Waitrose: LivLife low-carb bread.
Sainsbury's: HiLo low-carb bread, Oppo ice-cream, coconut flour and itsu Seaweed Thins, though not sure about those.
Tesco: The things that aren't pork scratchings but much lighter and easier to eat and are basically the same stuff. Their packets of 'Graze' nuts when they are half price. Also their version of Nescafe 'Azera' coffee, but Aldi's knock-off is also pretty good.