My daughter is doing a music degree at Cardiff Uni. Of the places she applied I think Cardiff was the only one which asked for an audition before offering a place, and it was very relaxed and friendly, I think she just played one of the G8 pieces she was working on and had a chat with the tutor. She was nervous about it, but found it fine, and Cardiff ended up as her first choice. I think Leeds asked her to do an audition at the offer holder day, but she already had the offer, so I'm not sure what the purpose of that was - maybe for the university orchestras/bands. I went with her to 4 offer holder days (Cardiff, Leeds, York and Durham) and they were all very friendly and welcoming.
My son had to perform at interview for Oxford Uni. It was a strange experience as the performance was separate to the other interviews. He said he was shown into the room where a handful of people were waiting. They did not speak to him. He performed (possibly guitar) and then they still did not speak. So he thanked them and left!
He was made an offer and is now an Oxford student. When he was little I used to throw things at him or shout out when he was performing for us in the lounge to prepare him for the unexpected! But we had never discussed what would happen in an audition where nobody spoke.