@hope your relatives...
So, if this guy was allergic to an ingredient in a drug, but didn't know it, presumably his GP also didn't know it, so when he was hit by a bus, the hospital, even if it had been able to get hold of his medical record from his GP, also wouldn't have known it. Seems to me a bit of a nonsequitur.
Personally, I'd be a lot more concerned about all and sundry being able to access my personal details in my GP record. At the moment, it's (officially) the GPs at my surgery and presumably, unofficially, receptionists, etc. if they still have paper records, being able to take a peek. If my medical records were to be made availabel to the whole NHS, this would open up that information to all 'authorised' personnel (i.e. doctors, nurses) throughout every hospital and surgery in the NHS, as well as any unauthorised access by admin staff, DB admins, hackers etc. that that entails.
When it comes to useful information not being available to hospitals when needed, this is an edge case for a number of reasons.
Firstly, anyone with a serious allergy or other condition should really either be wearing some sort of wrist-band or similar that medics can see to find that information, or carry that information about with them in their wallet, with their ID (which would be used to find the records in the first place). A friend of mine suffers from a rare metabolic condition. Rather than relying on his medical notes being found in an emergency, he has such information in his wallet. This has the added advantage that non medical people such as first-aiders can know about this, without his needing to be conscious at the time.
Secondly, in order to get a persons medical details, first they have to be reliably identified, and a search made for the correct medical details. At the scene of an accident, for example, even if a person can be identified at the scene, this presumably then entails the paramedic calling the hospital with teh person's details in order to get their medical details to knwo if they have any allergies before treating them at the scene. Not only does this add a possibly life-threatening delay, but also introduces the possibility of pulling up the wrong medical details. How many John Smiths will there be in a national database and how can you be sure you have got the right one? Most people don't carry their NHS number with them.
Thirdly, most hospital admissions whill be compis mentis enough to let the doctors know if they are allergic to penicillin. With those that aren't conscious, there is instead the problem with identification, so you are left with the edge case where those with mental impairment who know their own name but not whether they are allergic to anything are the only ones who this would help. Surely it would be a duty of care for their carers to ensure that such important medical information travelled with them as described above?