I'm sorry but this should be opt-in, no if's or buts. This is only for the government to make money selling our data and will make no difference to our care.
Claims about finding drug usefulness are rubbish, if you want to find that out then look at the prescription rates, high prescription rate = effective treatment. A doctor isn't going to keep prescribing a drug that doesn't work as well as an alternative.
Claims about targeting care are also rubbish, just keep track of referrals, they would actually be better than making assumptions about future care based on a medical record. Maybe go one better and invest in mobile hospitals for certain procedures so the care can be moved as and when needed.
I also get the feeling that this "opt-out" is a one time only event as I really can't see them setting up a system to scan for opt-outs when it isn't in their interest, there is also the fact that you wouldn't know or potentially have any way of finding out if your data was in this system. Furthermore thinking about the fragmented systems your GP data may not get in but how would you stop any records held by a hospital should you need to attend? Does anyone actually believe they would link your opt-out to your NHS number?