Gavin Jackson: To understand why Britain is run out of salad, it's best to think of a tomato as a kind of energy storage. Tomatoes absorb energy either from the sun's rays or they can get it some other way. The most efficient way of producing it is to put it in a field and let it just get that ambient sunlight hitting it. But when the sun's not shining, all the energy has to come from somewhere else. That often means burning fossil fuels.