It's the attitude towards the suppliers that worries me as much as the environmental thing. Having read a lot of Monbiot's stuff about how the supermarkets operate, it's shocking. They make their suppliers conform to ever more stringent specs, obliging them to do all the work for them in washing, packaging and labeling the fruit and veg for instance.
If they then decide not to buy whatever it is from the supplier that week (say because it's not selling so well) then the supplier is left with a huge investment in producing a Tesco-spec product and a warehouse full of food that's not going to sell.
All the risk is passed to the supplier, to the point where they're terrified of offending the supermarket for fear of being put on a blacklist. Tescos have even been know to approach suppliers for charitable donations to cover fund raisers that they are asked to participate in. If the supplier doesn't cough up, then...
I buy most of my fruit and veg from the "Ealing Show Market" on the Uxbridge Road. it's run by a bunch of Afghan guys who go and buy up the cheap stock at the Western International Market down at Heathrow. When the supermarkets reject the stock for whatever reason (not to spec) then it gets sold off to the highest bidder.
I eat whatever is good and cheap that week and it's amazing what he gets. When Waitrose charge £2.99 for a punnet of physalis, I can have them for 99p for six. Strawberries, asparagus, you name it, they get it. Sometimes I see him peeling the supermarket labels off the packaging. It's terrible.
Sure, it's been air-freighted into the UK, but the karma is the supermarkets' and not mine. I'd not buy it directly from them. I supplement it with UK grown apples, potatoes and other stuff that I buy in the Co-op and elsewhere.