Porridge is cheap and filling. You can make it with water or milk. Then add a spoon of jam or honey for sweetness.
I recommend buying salad veg and fruit from a market as it is a much better price.
Jacket potatoes are a great lunch, you can have it with baked beans, tuna (I think is expensive?)
It's hard to answer as I buy a lot of organic food which is expensive and I kind of forget what things cost. But you can buy what is on offer and use the following week.
Pasta is a great meal choice. Pasta, lots of veg, meat if you have some left over, pasta sauce.
I always forget to look in the freezer section at Tesco but they have some great offers on frozen veg and frozen meat. Good luck with weight watchers.