Here is some research on the food topic in this project.
Here is a good video of a Peter Griffith, an agriculturalist from Shropshire talking about the carbon footprint on food production. Carbon footprint in food production is about "taking more than you need". Food is 13-14% of the average UK residents carbon footprint including production, processing, distribution, packaging and the fuel it takes you to get to the shop to buy it.
Agriculture is 44% of food production emissions. The reason for this is a lot of methane and nitros oxide is released, 21x and 10x more potent respectively. It's a system which produces these which add to the carbon footprint by releasing even worse greenhouse gases than CO2.
Transport of food is about 16%, half of which is heavy goods vehicles. The food miles aren't much of a big issue, since the agricultural side is so much worse. Reducing food miles will have an impact but not as much as changing your diet. By being vegetarian you eliminate the methane and CO2 generated by cows and turning them in to beef.
