The policy world loves the notion of 'tipping points'. It's a simple idea. Rather like old-fashioned shop scales, with weights at one end that suddenly tip when the goods at the other end become heavier, so ideas and events can reach a tipping point when they become the new dominant force. Policymakers use it to focus minds. At what point will an idea or pressure tip the policy agenda? How can they capture events to deliver tipping points? How can they slow down or prevent tipping points?

I thought about this when National Statistics (NS) announced last week that, in 2004, UK spending on food and drink products consumed outside the home was greater than spending on food and drink products consumed in the home. By 2004, food spending in the home was £85.8bn and ­outside was £87.5bn. Spending outside home doubled between 1992 and 2004, whereas spending on food and drink in households only grew by half.

In a new report on the UK economy's inputs and outputs, NS also showed that agriculture's importance in the economy continues to decline. In 1995 it was 1.9% of the economy. By 2004, agriculture was less than 1%. In tipping point terms, agriculture has sunk. Many policymakers say it no longer matters. Farming has sucked on state subsidies for too long. Let it go.

The wider picture suggests caution is warranted. The net food trade imbalance has grown from £4.7bn in 1992 to £12.2bn in 2004. The gap between total domestic food-related output and imports is now a whopping £28.7bn. Within the food system, the gross value added (GVA) by the service sector (retailers and caterers battling over markets) has grown, while food manufacturing value has stayed stable. Agriculture's value has declined.

Most people, including NS, explain such shifts as driven by cultural change. But culture is not constant, which is why I was wryly amused at those insiders celebrating the recent survey finding that 61% of Brits don't think about food miles. Actually, a third of people are thinking food miles. Not quite tipping point, but getting there. Not bad for a notion that didn't exist in 1992. But I don't see a rebirth of agri­culture until other drivers kick in, such as fuel, water and climate change. This unsustainable food system has a few years of life left in it yet, I reckon. Any bets on the tipping point?