I too feel quite passionate about this having spent quite a lot of time away this year on various courses, short dets etc.
All my civvy friends find it outrageous that I have to pay for food when I am away from home for work, they all claim their expenses as actually but I have to pay for the cr@p that is served up at some PAYD outlets (have spent a lot of time with the army this year - including over a grant when there was little or no messing available) so why should we end up out of pocket for being sent away to work?
You shouldn't, end of. Furthermore, you should be given a flat rate (not outrageously high but sufficient to eat properly) in advance.