You can't claim directly for food..ie bought a burger etc, however, you can claim a Subsistence allowance if you are working away from home and take food with you for lunch.
Check with your accountant, he/she will know what you can reasonably claim. I don't know the exact reasoning, thats why I use an accountant. As a guide my accountant allows £2,600 a year for Subsistence for two of us out working.
Hope that helps a little.
Steve G