Hey Alberto,
Speaking from experience for the 12:45 appointment I would generally have a normal breakfast (if you count airline food as normal) and when you there ask to pop up to get your lunch there’s plenty available from curry’s to freshly made sandwiches. I arrived quite early 11:50ish so I got my eyes tested then popped up for lunch but I did refrain from going to the toilet after I got to the airport.
Just eat and drink what you would normally (eating porridge a month till your date before hand should reduce your cholesterol) what you would normally.
The Dr