So he said, when put on the spot, that they spent 70-80 per week, probably more than that. So how long is a piece of string? If they all trundle round Waitrose once a week and throw everything they want in the trolley, what does that cost? 150-200? as a complete guess.
He (and his DW too?) are professionals working long hours in highly paid jobs. I would expect that they have some help at home and perhaps do something like have a regular Ocado order to keep the fridge full. There's probably a lot of eating out for lunches and functions. May be they have someone who goes and picks up bits from a local supermarket once or twice a week and that is genuinely what it costs? As well as being leader of the Labour Party, Ed Milliband, is a normal person with a normal household and will have a personal bank account that someone will need to keep an eye on. Perhaps he does that himself?
You can only eat so much food, Just because he earns 400k per year, doesn't mean they spend 10x as much on food as someone who earns 40k per year does it?
It's perfectly possible to feed a family of 4 very well on 70 to 80 per week. OK, that amount of money won't pay for the trundle round waitrose with no restrictions on what you want, but it will pay for sufficient meat, fruit and veg etc for a healthy diet, providing that there is an element of cooking from scratch (which is cheaper, despite many people refusing to believe this) and use of seasonal foods. There are plenty of people to whom 70-80 per week for a family of four would be a fortune.
The real avoidable expenses in the grocery shop are things like fresh fish, out of season fruit, branded cleaners for lots of different purposes, posh ready meals and alcohol.
There will be a huge amount of variation in spends on these items, from next to nothing, to tens of pounds a week or more, and not one of them are essential for a healthy diet of quality food.