I love love LOVE Marks & Spencer, but they can be a bit pricey, so it's probably best not to buy staple items there, like ketchup, or fruit/veg because you can get basically the same stuff at a supermarket chain, for a lot less. However, they have some LOVELY ready-meals, delicious, LEAN packets of cold meat, and scrummy desserts. I always used to balance things out budget-wise by shopping at a variety of places, so I could get everything I enjoyed without breaking the bank.
I agree with what Peedal said about Sainsbury's -- I think that they're own-brand items are on the whole, very good quality and can definitely be trusted. Safeway's own-brand things can be quite good as well, like chip dips and some of their ready-meals. Tesco, ASDA and Somerfield's can be hit or miss. I pretty much only shopped at those supermarkets if there was an item I wanted that was on sale, or if I needed something in a pinch. I didn't discriminate too much on things like paper towels or bin liners, but for me, dish and laundry detergent needed to be name-brand.
I think across the board, though, everyone has different tastes and really the only way to figure things out is by trial and error. I must've tried about 8 different kinds of raspberry jam before I found one that resembled my American favourite, Smucker's. (Bonne Maman was the closest!). If you just allow yourself to be flexible and open to trying new things, it will help a great deal.
With reference to the butter issue, I always bought Anchor Spreadable Half-fat, and my flatmate bought Flora margarine. I can attest to the fact that both are good!