I (controversially) do most of my supermarket shop in waitrose. Not the obv choice for cheapness perhaps but they send me loads of vouchers, e.g. I had £15 to spend this week on anything in store (no minimum spend), a free massive toblerone with the same shop as I spent £40, then I have a voucher for 10% off every shop from now til 7th July.
Plus if you put it on a John Lewis card, then clear it (not building up debt again!) you get loads of JL points which convert into more vouchers you can spend in Waitrose or John Lewis.
I usually get things like: mixers, pasta, soy milk/almond milk (allergies!), flour, eggs, sugar, jam/peanut butter, margarine, some fruits, toilet rolls etc and find their waitrose 'essentials' brand is very competitively priced.
We get a local veg box so usually only buy top up veg in waitrose e.g. onions for a recipe etc. Also use a local market on weeks off from veg box and you can really get some eye opening bargains - for the past three weeks we have gotten a massive 1.5-2 kilos tomatoes for £1. Gorgeous, red ripe ones too.
btw I am a teacher, I am in no way affiliated to waitrose!!