What can they do? Taking your home made sarnies off you has to be theft. Stopping you eating them by force has to be assault and battery. So many companies are incorrectly using "health and safety" and "data protection" to justify rubbish customer service. Just ignore them and take your own food if you want to. If it's not a flight safety issue they cannot tell you what to do unless it specifically says they can in the law, like stopping you drinking your own alcohol on board. No law says I cannot eat my own bacon, lettuce and tomato sarnie on board a flight, no matter how much mayonaise it has on it, and no matter how long it's been at my feet.