I learnt my lesson some time ago when returning one night to the UK on a "low cost" carrier. I was starving but there was no food left for sale. Since then I've always made sure that, if at all possible, I eat on the ground. It can be cheaper, too: compare the price of a sandwich from Boots at the airport with one onboard a Ryanair or easyJet flight.
P.S. I'm also somewhat suspicious about how long things like mayo can remain safe in the cooler bags some airlines use. I get the impression that these are loaded once a day, before the first flight. Why don't more airlines use fridges? (Weight, I suppose.)