Maybe I'm just old but my pertinent medical information is written on the appropriate page of my passport and on a piece of paper kept with my driving licence counterpart. My GPs number is in there too should some emergency medical staff want a chat.
This system seems a little less invasive and less likely to be abused than some of the high tech options being suggested here.
Hungary issue I.D. cards as do many EU countries
http://en.wikipedia.org/wiki/National_identity_cards_in_the_European_Economic_Area For one thing I could leave my passport behind when travelling within the EU.
Biometric chips on the card (and I'm very pro a UK wide DNA database) would save time and lives and the DNA database I've no doubt would reduce crime or certainly help solve many and I'd go so far as to add fingerprint I.D. to the card.
I really don't understand the UK 'Luddite's' who are so against I.D. cards given the amount of personal data so readily available to so many UK agencies.
Medical braclets et al are very often ignored by emergency medical services...they can't be sure its 'you' and as for phoning your GP then good luck with that one after hours.