We spend on average 40 pounds a week on food.
I started tracking all of our spending last November, and it took a while to work out what our average is, since we sometimes have months where we'll do very large shops and stock up, and then have months where we're hardly around, and sometimes I'm better about brining my lunch to work, or making big batches of meals to eat throughout a week, etc, etc. I would start tracking exactly what you are spending, but give it some time to show consistent patterns.
We don't buy a lot of convenience foods or snacks, and besides wine/beer (which we stock up on a few times a year), don't buy any other drinks since we prefer water.
Trust me--I was equally surprised when I figured out what I was spending last November. But even just keeping a record of it has helped my husband and I cut back, as it makes us think twice.