I gotta say, as a single student who doesn't want to pay 10 pounds for a meal every night (but is also too impatient to cook), this Sainsburys has been a lifesaver.
The multitude of last-minute, healthy, pre-prepared meals here is exhaustive and has gotten me through many a late night (where I desperately try to find food after 10pm only to find every other store around closed other than the area's many fried chicken shops which, inexplicably, seem to be open 24/7!)
As someone who tries to eat vegetarian as much as possible, I've also found the veggie-meal selection to be quite good - and there's an aisle of fresh produce veggies that's also satisfactory for anyone on the run looking for something nutritious. I can choose between a variety of salads chock-full of grains, nuts and healthy vegetables, pasta salads, fresh shrimp combination meals, indian masala portions, tapas, stews, veggie pots, etc - all for bargain prices (ie, under 5 pounds per meal).
My recommendations as a Sainsburys pre-made meal connoisseur (ah, the joys of being a student)? 'Innocent' pots (full of vegetables and vegetarian options, with cute recyclable labels and a variety of flavours at 3 pounds each), the vegetarian/shrimp Indian platters (complete with jasmine rice, sauce and veggies) for under 5 pounds, and the quick-grab sandwiches, most sold for under 4 pounds and filled with tasty veggies.
My last thumbs-up? The self-check outs. This way, when I'm too tired/rushed to deal with a human (which happens often), I can check myself out and hear an (oddly soothing) android voice tell me thank-you-for-shopping instead ;) read more