I wore the same for dip and LRSM – smart black trousers, red jumper. No evening dress. The exams take place during the day, so it feels a bit weird to turn up at 9am in a sparkly ballgown, especially with two examiners stationed behind a desk and never once applauding. Also, I wanted to keep warm. The centre I played at was freezing cold and had fan heaters going in the exam room.
Dress is the least of your problems. It doesn't matter that much (though I'd avoid ripped jeans). Stick with the lunchtime concert look.
Jess Gillam wore a frilly mauve evening dress for her early attempt at Young Musician. She'd completely changed style by 2016 and it suited her much better.