As long as people are informed, they should be able to make the choice themselves about what happens to their own body.
I am personally of the view that the system should be opt-out. But I can appreciate the arguments against.
In China, people on death row frequently have their deaths hastened so that their organs may be harvested. Prisoners provide most of the organs for transplant there. There have been many cases of families being unable to say goodbye because their loved ones are killed off so that the organs can be taken.
For that reason, I'd never make donation compulsory, and I'd be a bit hesitant to make the system opt-out because of the fact that the vulnerable and uneducated would not necessarily have an informed choice about what happens to them.
Obviously the UK isn't China, but it's the end result of seeing bodies as just sacks of meat. When we die, we leave our bodies. But the bodies are buried, cremated and put into urns, cremated and scattered. Even bodies donated for medical student dissection are finally cremated with everything put back inside, as "intact" as possible.
There are deep spiritual reasons for people wanting their bodies to be whole. Especially bodies of children.
I understand the pain of a person whose child may die on the transplant list, and it is awful. But does that give them the right to take an organ from the body of a child another family has lost, if it will cause that person pain and deny them closure? I don't think so.
Everyone should be informed and make their own decisions. People dying on the transplant list should not be put on the shoulders of the bereaved, especially bereaved parents. That is not empathic, not respectful, not kind at all.