This is from the article in the Independent today:
"Next time they are at the supermarket, green enthusiasts may want to think twice before opting for British strawberries and tomatoes over those grown in Spain. British-grown varieties fare badly compared with Spanish imports in terms of energy use and global-warming potential. Much more energy is needed to heat greenhouses here – not an issue in sunny Spain – so there is a trade-off between the increased use of gas and electricity, and the longer transport distance and greater demands on water in Spain.
"And if transport is taken out of the equation, lamb from New Zealand is a more sustainable choice than that farmed in Britain – with less energy used for farming in a climate where there is less need for feed supplements and heated farm buildings.
"If Britons want to continue to have a choice of food all year round, it may be better to import some staples and avoid the emissions otherwise caused by keeping British produce in chilled storage for long periods, according to researchers."
So DEFRA are saying that if we want strawberries at Christmas then we better buy Spanish ones as the combined energy of their greenhouses and shipping them here will be less than if you grow them in the UK. How exactly is that complicated?
The key of course is seasonal. Of course local food won't have less embodied energy if you are trying to grow Kentish pineapples in winter. My early summer strawberries, grown in the back garden not only have the lowest possible food miles but require no heating at all - and they taste much better than shop bought ones too.