It's only me and dd in the house, but I don't spend anywhere near £50 per week. It's probably more like half that plus another £25 per month on non-food items (washing powder, fabric conditioner, shampoo, cat/dog/rabbit food, etc.)
I do the majority of my shopping in Lidl. They do a 65 wash powder (non-bio/bio/colours) that is just as good as Persil and I picked one up a few weeks back for £6.19. It will last forever even with me washing like a demon. They do a gigantic box of dishwasher tablets for just over £3.00 and I think a spray bottle of antibacterial cleaner is about 59p. I buy only organic/fairtrade fruit and veg out of there which is very cheap and lasts. I recently stopped eating meat but do still eat fish, and their fresh fish hardly dents the budget at all.
For frozen fish, quorn products and dd's slummy mummy meals (fish fingers, chicken nuggets, etc.) I go to farmfoods. I also buy my toilet roll from here (Nicky quilted stuff - 18 rolls for under £4.00).
For all animal food I go to Tesco, where I also stock up on value tinned tomatoes and the like. I make a special effort once every two months to walk up to my local Pound Stretcher and buy several packs of tinned tuna - John West or Princes 4 packs are under £3.00 each.
A typical food week for us would be:
Breakfast: Cereal / porridge with raisins / toast and banana / omelette and toast. We drink organic milk and usually have a fruit juice of some kind as well.
Lunch: Sandwiches using wholemeal bread / homemade soup with chunky homemade soda bread (Jack Monroe's recipe which costs pennies) / cheese and crackers with grapes and homemade chutney / salad with cheese and tuna mayo.
Dinner: Spaghetti bolognese using heaps of veg and quorn mince / smoked haddock chowder / quorn sausages with veg, gravy and a big ol' Yorkshire pud / cheesy wholemeal pasta with veggies / jacket potatoes and salad / pizza, garlic bread and salad / salmon fillets cooked in a little oil, butter and cracked black pepper with veg / smoked mackerel salad.
Snack: Fruit, carrot sticks, yoghurt.