Cheap loo roll, not only do you use more but no amount of money saved compensates for that vile finger-through-the-paper experience.
Cheap bleach, it's like water so you use twice as much. Own brand, yes. Value, no no no.
Value tinned tuna, it's just fins, bones, and brine. Ditto value tinned salmon.
Ready-made dinner kits like the Old El Paso fajita kits, etc. It's usually cheaper to buy a pack of tortillas, a jar of salsa (or some tomatos and chilli), and a jar of mexican spices.
Also yes to cheap meat like value packs of mince. It's snouts and hooves. Cheap cuts of meat however are excellent for slow cooking, boneless chicken thighs practically melt in a slow cooker curry.
Key meters for gas/electricity. In our one bed flat when it was just the two of us DH and I were spending upwards of £60 a month on the gas and electricity key meters. In our two bed house with two DCs on a tariff we're spedning £49 a month.
Cheap nappies. I know some people will disagree and some cheap brands are okay (Lidl own brand and Kiddicare Just 4 Bums are great) but Little Angels and Simply Dry are the pits. Whenever we've used them on DS he gets nappy rash because they don't lock away moisture as effectively and we get leaks so we spend more money on nappy rash creams and doing laundry.
Primark jeans. I find their children's jeans tend to shrink after the first wash this may be down to my rubbish laundry skills though