As others have said, not a formal complaint, but a letter. However, as has also been pointed out there can be good reasons for needing everyone there to start with.
When I needed minor day surgery on my stoma, it was me that pointed out to the Consultant previously that as there were no nerves there I could have it done without anaesthetic.
Caused my own chaos with that, when it was finished, nurses kept trying to prevent me leaving without someone accompanying me, and I kept pointing out that I did not need anyone as I had received no anaesthetic.
When I went first thing in the morning, and realised I was not going to be done until afternoon, I asked if I could go back home to wait. As the Sister in charge knew me (I worked at that hospital), I was eventually given permission, being told to return by Midday. When I did return I queried why they had been so reluctant to let me leave for a couple of hours and was told it was because, when they had done this with other patients, they often did not return.
Yes, they do expect, and have to take into account, a lot of 'no shows'.
Nothing to be feared about with day surgery, Take a good book, and a bottle of water. Unless you have some sort of condition like diabetes it really does none of us any harm to go a few hours without eating.