The UTag is a memory stick, stamped with a medical symbol that can be used to store personal medical information which will be useful to the emergency services if you are involved in an accident. A newspaper report by Louise Bolotin in the U.K newspaper The Guardian points out that this is a more efficient way of carrying medical information than clunky old-fashioned bracelets.
I suspect that emergency services consider these devices and the ICE (In case of emergency) phone numbers as a low priority at the scene of an accident. It is when you have arrived in hospital and have been stabilised that this information will be used.
But why limit the amount of data to allergies and next of kin?
The storage capacity of memory sticks increases all the time and the cost is falling. Why not store all your medical information on a personal stick and back it up to a secure location on Google? See here for a previous post detailing this suggestion.This would allow you to be in control of your own confidential medical information. Privacy would be improved and with present capacities of 8Gb even the hypochondriac will be able to store all of their medical history.
Since the NHS are incapable of delivering a working integrated computer system perhaps it's time to, literally, hold our own records