The first time John does the shopping and doesn't pick up a single own - brand item, it's a momentous occasion. It's not, truly, but John tells himself it is.
His family wasn't poor. His parents were working class, and proud. John never felt he'd missed out on anything important just because he grew up eating Sainsbury's corn flakes rather than Kellogg's. "More for the money," mum always said, and it made good sense. Especially in uni, when he couldn't always keep a good job and good scores.
Army rations weren't bad. They weren't necessarily good though either. But John didn't have to pay for them (he'd realize the error of that thinking halfway through his third tour), so anything was better than nothing.
When John returned to London with nothing but a scar and a tiny pension, the shopping wasn't a question. The shops sold nearly expired or slightly damaged goods for a bargain. He learned to make do. Just barely.
But the day he picks up the Heinz beans instead of the Tesco ones, and walks right past the shelf of discounted dented cans, is a good one. He's been at the surgery for six months now, and Sherlock's had three paying clients in as many weeks. The rent is current and the utilities are paid. John splurges and picks up the chocolate McVinties Sherlock likes. He doesn't even fight with the chip and PIN machine.
An overnight chase through the city, and the shopping is forgotten until the next morning when John decides on beans and toast. He offers to fix Sherlock a plate and is met with silence. John takes that as an admission of hunger since refusals are delivered with some vehemence.
He places Sherlock's plate next to his elbow, and doesn't realize it's even been acknowledged until he sits down across the table, his own plate in hand. Sherlock has taken a bite and is chewing thoughtfully. He manages to swallow before he scowls.
"These are terrible. What have you done differently?"
"What? Nothing! They're perfectly good beans. I just got them yesterday." John furrows his brow and examines his food. "Did you spill something?" He waves his hand over the mess surrounding Sherlock on the table.
"Of course not!" Sherlock is indignant as he turns and dumps the plate in the sink.
John hums in frustration and takes the first bite of his own breakfast. He finds himself frowning at his plate. "Oh god. That's awful." Sherlock casts an imperious glance his direction before returning to his microscope.
The next time John does the shopping, he replaces the Heinz beans he's agreed to let Sherlock grow mould cultures on with the dented Tesco ones.