She's 18 and you want her to go away to university. You're unnecessarily strict and setting yourself up for confrontation - presumably because you don't like her boyfriend.
At 18, I would say that the rule is if you live here and you're not coming home one night, you let parent know by so that they don't worry about you.
Slouching round the house: my rule would be not to create any work (so - she does her own washing) but otherwise her time is her own. If she doesn't have a job then it's her without any money - no skin off your nose, really.
As to meals: if she has told you she's in, you feed her. If occasionally she then changes her plans, I'd plate up her food and stick it in the fridge for her. But no confirmation she's in, no dinner made. And if it was a constant change of plans AND the plated up meal wasn't eaten by her next day and wasted, then I'd stop including her in the meal.
She can "expect" all she likes.
I wouldn't fight her staying with her boyfriend - I don't see why that has to be a problem.
I would expect to feed my own child, so fridge raising for normal food (a cheese sandwich at lunchtime) would be fine. Fridge raiding that meant eating all the lobster meant for a family dinner - no!