Its quite easy to feed 5 people for 50pounds a week, especially if that doesn't include toiletries, cleaning stuff etc. I think mainly that you have to manage expectations, really and be prepared to use tinned salmon instead of salmon fillets, for example.
The way that I work is to meal plan everything, even down to snacks and drinks. Then I shop online so that there are no extras grinning at me from the aisles. If I find a very good special offer, I might change my plan, but most of the stuff that I buy tends not be affected much by offers. I buy quite a lot of tinned tomatoes, for example, but I know that they will almost always be either 2 or 3 tins for a pound, so I just check which is the best buy every month. Occasionally, if I am in a shop, I find a really good whoops thing to buy, a leg of lamb for a fiver, mince for 50p or whatever. I will buy that and freeze it, but then the next lot of meals are planned round it and that cuts down on the next shop.
The easiest way to save money is to plan meals around the carb and then fill in, also to make more one pot meals instead of meat and 2 veg sort of meals.
So you could have 2 rice/other grain, 2 pasta, 2 potato and 1 bread based main meals a week. Then work out the protein that you need for these. Try to make at least 2 meals from beans or pulses, then another eggs or cheese, and one fish (I use smoked mackerel a lot because it is cheap and you only need a little, tinned salmon is also a good source of fish oil as are tinned sardines and pilchards. Frozen white fish is perfectly adequate for pies, fish cakes, soup etc.)
Another good money saving tip is to cook some kind of roast once a week, strip all the meat from it and divide it into 3 or 4 meals worth. I NEVER serve a roast as a roast dinner, it's just not cost effective. You can then boil any bones up to make stock for lunchtime soups. Also try baking as much as possible at home. HM bread, cakes, biscuits, scones etc. are much more cost effective and really easy to make.
So you could cook a chicken, for example and them have:
creamy chicken and veg jacket potatoes (use 1 breast shredded, about 1lb of mixed veg of some kind, a couple of onions and 1 1/2pints of white sauce using 1/2 milk 1/2 chicken stock from a cube)
spicy chicken tortilla bake ( other breast finely chopped mixed with a tin of tomato soup, a jar of salsa or a tin of tomatoes with chilli and peppers and a tin of chopped tomatoes, mixed with a 7 or 8 tortillas cut into 1inch squares. Pour into a pie dish, sprinkle with grated cheese and bake for 30 minutes)
Chicken risotto and salad (use scraps of meat)
The you could fill the rest of the evenings with:
smoked mackerel kedgeree (use 3 eggs and 1/2pound of fish)
tomato and cottage cheese pasta bake
lentil lasagne
lentil and veg shepherds pie.
Then you can make stock from the chicken carcass to make soup for lunches and use the rest of the meat for sandwiches, savoury muffins or little pies or pasties .
LAst week I cooked a reduced lamb leg and used it for 4 meals and stock. I made barley mince, lamb and veg tabouleh, lamb rissole things and shredded lamb wraps.
Like India, I don't waste anything. Little bits of things are recycled, dying stuff gets turned into soup, cakes or whatever or pickles, chutneys and jam. I also buy multi purpose ingredients. Lentils can be made into soup, chilli, curry, loaf, burgers, pate...., dried beans can be used in a hundred and one different meals. A small pack of sliced chorizo can be use to make patatas bravas one night, then the other half can be used to make chick pea and chorizo soup (you only need a couple of ounces to flavour a whole dish.)A tin of salmon can make fishcakes, pie, pasta, fritters, loaf, burgers, sandwich spread and more. Things like that mean that the family get a wide variety of meals from a limited variety or ingredients. So they don't get bored eating the same things all the time, but I don't have to spend an unsustainable amount of money to feed them well.
There are a couple of other threads on this topic about cutting back on shopping bills that are full of good tips. There is also one on Good Housekeeping just now, called Back to Basics I think, that's about this as well. You could have a look at those.
You could also try here. The recipes are good and very cheap, but you do eat the same thing over and over again if you stick to the plan completely.