Cost effectiveness of food boxes
Supermarkets in the UK unsurprisingly have massive buying power and therefore one would assume that they can offer some seriously cheap vegetables. However the truth is, is that food box schemes are consistently cheaper than the major supermarkets. The big supermarkets will buy from the growers at the lowest possible rate, often squeezing them, and from anywhere in the world. By selling direct, the producers can offer their goods at an even lower price which is greater than what the supermarkets would give them.
Quantity also comes into it. Small farms with a limited capacity would perhaps be of little interest to the super market buyers but are more than capable of fulfilling the needs of local demand. The small producer is more nimble, able to respond to supply and demand and where appropriate, diversify away from their normal crops.
It is in the interest of the small scale grower to be competitive, and offer their produce at a lower price than can be found in the supermarket. As such, most check their pricing against the supermarkets on a regular basis to ensure they maintain their competitive edge.