Beautiful dress. I've worked at over 100 graduation ceremonies at a very high ranking and well respected uni, and I've seen far more revealing and scandalous dresses in the last two decades, this wouldn't make me bat an eyelid. It won't be massively practical with the gown but I wore a borrowed frumpy blouse for my graduation and hate how I looked! I wish I'd ignored the advice about needing a buttoned shirt and just wore something I felt comfortable and attractive in.
There's not really any impression you need to make or image you need to convey, it's a day of celebration, not a job interview. If there is no formal dress code then I'd suggest something you like. If you can't do that when you're celebrating years of hard work and you're (presumably) in your early 20s then when can you.
I wish I could afford a £300 dress for my graduation as well haha. I didn't have two pennies to rub together, hence the borrowed blouse (and it really was a frumpy blouse, not a cool neutral shirt).