When I end up in A&E I'd like it that the people dealing with me could
have an accurate record of allergies, medications and so on.
This is exactly the sort of thing that the SCR is being developed for.
Another thing that's worth mentioning on here is that 'opting-out' isn't meant to be an all or nothing proposition. The system is designed so that you can include or exclude various bits of data depending on what you feel comfortable sharing with GP's outside of your registered practise. I'm presuming that this is why the literature says you're supposed to bring in your opt-out notice in person. It's supposed to be an interactive engagement with your GP where you decide what is considered sensitive (mental health issues are the example we use around the office all the time), and what can only be considered helpful to publish (such as drug allergies).
There's also the option of "add to my SCR, but require permission to access it". In fact, I'm pretty sure the SCR spec says that this is meant to be the default access permission on all core SCR elements until you say otherwise, but I'd have to double check to be certain.
