We also get quite a bit of our shopping from M&S, as it's not as expensive as you might imagine.
I saw a post a few weeks ago about big blocks of very good value cheddar. Also milk, eggs, basic salad and veg like lettuce, cucumber, carrots, potatoes, mushrooms etc, some meat such as the higher welfare Oakham chicken boneless thighs (or we sometimes get these from Ocado as they sell larger packs with quite a good saving per kilo, I've never seen these in M&S stores), the 'family meal deal' and Dine in type deals, the stir fry deal, bread and of course anything reduced. Their canned soups are good. I don't like Heinz and their tomato soup is more like Baxters, which I do like and it's only 55 p.
On pricing, a lot of things have been held at 'round' pricing like 50 p, £1 etc as a loss leader and they've probably held these for as long as they can, but now they can't, they've priced them at what they need to sell them at, hence huge and sudden price rises like £1 to £1.60.
I think it's sometimes possible to mitigate the price rises a little, but it requires flexibility over brands and an ability to shop around. We've never done a regular weekly shop and have always rotated round a few different places and stocked up when in shops where the things we like are cheapest, so we rarely 'need' anything from a single weekly big shop, so if it's more expensive or not available in one particular shop, it can be left until the next shop.