Also, just on a human level, people don't want to be chopped up and parcelled out as used parts for other people. I don't feel that way, but I understand how people can feel that way and we should respect their wishes.
...in which case it's easy to opt out, for the people who do feel that way.
But they can't if they don't know it's opt-out, which is why thread OP argues that BBC should announce this.
I don’t agree that their irrational preferences trump others’ right to live, but let’s assume they do. In that case, they should automatically forfeit the right to receive transplants. Not just be put at the back of the queue, but be banned altogether.