I downloaded the app Flash Food. I haven't used it yet but it looks interesting. Right now I think the only store for London is Farm Boy. Basically you can buy products that are surplus or will expire soon at a reduced price. Some deals are ok, some are really good deals. This would especially be good for larger families that can consume the product quickly. So far I find it's mostly yogurt and milk products but you will also find baked goods, prepared foods... right now you can snag a box of crackers for 99 cents. Some pictures show the expiry date and in other instances you can get an idea by how long the product is selling for. A couple of times I've seen 4L of Natrel Lactose free milk for 3.99 selling for up to 7 days.

You buy buy through the app and then go pick it up in store.