I do not like the state owning people's bodies which an opt out system creates.
It puts those who are particularly vulnerable in a position which I find worrying. Especially in a climate where health care is becoming much more rationed and with doctors in a position where they have to chose between patients.
A patient who has no family, has language barriers or has some sort of learning difficulty therefore is in a position where it is possible that they are not given health care of the same level as someone else who is privileged because they become percieved as of more value and importance as donors rather than having treatment. They are less able to argue or have someone advocate for them to continue to have care.
I find an opt in system far less problematic in this way and vulnerable to conflicts in interests and NHS rationing because there are no guarentees that organs will be donated.
I'm very much of the 'presumed consent is an oxymoron. If it's presumed, it isn't consent. As a concept, introduced in law, it's very worrying.' school of thought