I have opted out as I am donating my body to medical research. There isn't a section to explain your reasons for opting out though.
Very few people are actually able to do this, sadly. Massively depends on why you die, where you live, whether you die on a weekend.
My mum really wanted to donate her body. She died from a very rare cancer. Nowhere wanted her. It’s sad when you think how much could be learned from those who die from illnesses.
Unless you die on a weekday and are reasonably healthy with normal anatomy, it’s actually very difficult to donate your body to science.
She couldn’t donate any organs either - the only thing she was eligible for was donating her corneas but that was out because she’d had laser eye surgery.