It's all very well costing £2.50 of nutritious packed lunch from a particular supermarket, but a lot harder to do so from what's available in the only shop within walking distance which in many areas could be a Bargain Booze Extra, or Shell garage.
Everything else aside, this is such an important point. I did some qualitative research on this a couple of years ago. Supermarkets rarely open large stores with cheap offers in tightly packed and poor residential areas. They open them in large spaces that require travel to/from which often excludes the people who would benefit most from being able to shop there.
Obviously that's not always the case, but many impoverished areas are also food deserts, and it can be hard to find anywhere other than the corner shop to buy groceries. They are not selling at supermarket prices.
This is an article on obesity, but there's a section that covers these issues quite neatly:
www.newstatesman.com/culture/culture/2013/05/our-big-fat-fear
Also, the 'I've done this, so anyone should be able to' argument is, I feel, a bit specious. I've done hard things that other people haven't - most notably leaving an abusive relationship - but that doesn't mean I blame them for not doing it. Their mileage clearly varies, for whatever reason, and sometimes people need more support than others. It's not a moral failing.
Yours,
A bleeding heart liberal.