"...does not guarantee delivery of SMS messages'." That's a major problem with SMS messaging. Most texts are sent and received quite quickly, but now and again, I've known a text to take upto an hour to travel.
The second link to Ofcom from fourm member is much more informative, which makes the emergency SMS link homepage a bit of a failure in the first place bearing in mind "...because most of us rarely have reason to call 999, the trial needs thousands of people to register so that there will be a sufficient number of 999 messages to test the system."
The Ofcom link sells the idea much better and slao suggests why registration is required... "People who send hoax messages can be barred from the system and may be prosecuted."
The only problem I can see is how do you text for an ambulance, if your emergency is that you've broken both thumbs...?