Mysupermarket is great for finding the best deals on branded crisps and so on. I only ever buy crisps on special offers and we are working on not eating the entire multipack in the first 2 days! If we do, they don't just get replaced anyway. FarmFoods has some good deals on branded crisps too.
Meal planning can work however you want it to. When doing a weekly plan, I started on the basis of 1 fish evening meal, 2-3 meat and 2-3 veggie. I don't count Sundays because we have shared meals/catering at church. Sometimes I start with say a whole chicken (because I know there is a good offer on!) and plan a roast, a pasta meal with the leftovers and a risotto with stock from the carcass, and maybe it'll make one day of sandwiches too. Then because I get parsnips and sweet potatoes to roast with the chicken, I'll plan in a parsnip soup with the rest and orange mash the next week with another meat meal.
Sometimes I decide we're due a treat and get something delicious like lamb steaks (because I see on mysupermarket.co.uk they are on special offer obviously!), in which case the other meat meal is a cheaper sausage&beans casserole or a curry with a minimal amount of Basics chicken (freeze the rest of the pack). The fish is dependent on any offers as well, but I would only have salmon on a week where all the other meals were cheap. Otherwise it is frozen breaded fish & homemade chips & veg, or smoked mackeral (half the pack in a pasta meal or curry and the half made into pate for lunches) or a tin of tuna or salmon made into fish cakes. Veggie meals include soups, really basic pasta bakes, homemade pizza, noodles, curry & rice. I try to balance the carbs over the week as well so we only have pasta 3 times max!
As you get used to doing it, you start having patterns of fitting ingredients together over the week. Say buying a bag of peppers, having a pork goulash with 2 of them, a stirfry&noodles with one, one for homemade pizza and one for lunches. Or seeing spinach on offer and basically having it in nearly every meal to use it all up!
But I definitely find planning the month in advance more useful/frugal - otherwise I forget to plan in what I have in the freezer or in the stock cupboard. I can also plan in to double-cook to freeze meals for the next month or whatever, based on some expensive ingredient on special offer.
Oh - and home-made houmous with tinned chickpeas is pretty cheap! One (25p on offer for Eid) tin makes loads! Fresh olives are a rare treat for us now sadly, but the bottled ones used frugally last ages.