The danger will be in opting out
If you opt out, then the govt/nhs have to keep your records in a separate, secure confidential database, away from the more readily available one.
So you're guaranteed that someone will leave a copy of your health records unlocked on a laptop or usb drive on a train or in a taxi every 4 months. Whereas, by staying opted in, your data should be safer by obscurity, as it'll be lost amongst all the rest of the populations data which will be held in a privately run database with the correct levels of access controls and redundant backup systems.
Possibly, they've spent the last 5 years consistently "loosing" people's private health data in order to convince us that being auto-opted-in really is the better and safer choice.