Out of Synch,
How do you get to eat your meal just now?. The same would apply if you brought your own. If you have time to eat a supplied meal you will have the same time to eat your own supplied meal.
Not all workers get time off to go and shop during working hours. Bring the sandwiches etc with you and keep them in a container that you can buy in any high street store and they will keep. If you leave your meal at home buy some at the airport just like the passengers do.