pete999 is right, if you have been provided a place with cooking facilities it could be considered reasonable that you will cook your own meals the same as you would at home.
don't be greedy is the key to the whole thing.
however, the general rule of thumb is that you should not be out of pocket for working away.
if you are on a 3 month assignement away from home and you are provided with a place that has a fridge and cooker, I would not expect anything for food etc.
but anything short term, I would be pulling the REASONABLE EXPENSES phrase out of my hat and expecting to be fed and watered.
I am self employed now and generaly charge my customers £60 a night for food and lodgings if I am outside London. this gives me £45 for a travel lodge and £15 for dinner and a beer.
but back when I was employed I would expect the company to put me up somewhere and feed me. if they failed to provide for my reasonable costs of doing so, I would have been unwilling to do it again.
I think I have been both fair with my previous employers and possibley lucky that I have had no problems with a disagreement over what is reasonable.