if you can address balance there are very few banned foods. I tend not to miss meals where possible, because I need the fuel and training suffers if I do.
If your purchased lunch consists of 2 chocolate bars, a salad where the sauce weighs more than the veg, and a deep fried sausage, you may struggle to get that into your daily balance but if it's sensible you are unlikely to suffer too much.
A good tip to ensure you bring food. Don't bring money, that will motivate you to remember. I never said it was a nice tip, but I'll bet it would happen less often if you went hungry once or twice.