I am not sure whether we say to have "milk and cornflakes" or "milk with cornflakes". Could you give me a hand?
Much depends on the context. Here are some possibilities:
1. I'd like cornflakes for breakfast, with milk.
2. I'd like milk with my cornflakes, please.
3. Could you pour some milk on my cornflakes, please?
4. I'm eating cornflakes with milk and sugar.
"I'm having milk with/and cornflakes" sounds slightly odd; milk is the accompaniment to cornflakes, so naturally follows it.