I think it is important to be there when the results come out. We got results in school (abroad) and I remember doing not so well in one of my subjects. I was in total shock and first thing I did do was call my mum and have a really good cry. She thought I had failed everything and was really shocked but incredibly supportive.
I figured the rest out myself, re-sits etc, but it was good to have her there. And while I went out that night with my friends, that afternoon I went home to have a nice relaxing day with my mum, another good cry and some comfort food.
When I got an A in an important oral final I called her afterwards and she screamed down the phone, she was so excited. Everyone at work heard her. That night we had a mini celebration too.
She helped me so much to get through school, was there for 13 years of education, helped with primary school projects, picked secondary schools and majors with me, studied through the night with me preparing for exams... I know that she was just as happy/devastated as I was on result days and definitely just as much on edge.
I was very independent and left for a year abroad shortly after all of this. But then I don't believe needing support from ones family has anything to do with being an adult. Sometimes we just need that extra bit of support, even if it's just coming home and getting a hug, having someone bring you tea when you make life changing phone calls or whatever. I don't think we ever not need this, be it 17 or 45. When your future depends on some results, decisions etc it's nice to have support.
So OP, I think it is nice you are going to be home that day and please don't be angry at your daughter. As everyone has stated this year is so hard for people getting into uni, she must have felt the pressure all year. I am not surprised she got angry when your DP told her, I think we can all understand overreacting sometimes when being on edge, no?
Do tell you DP though that you would have loved to go and think you should post pone the trip. it is lovely of him to do this and it sounds like you had a really hard time, so you deserve a lovely break!