I was under the impression that you COULD dial 999 if you didn't have your network coverage but were near other signals; the last three mobiles I've owned over the last six years or so read "Emergency Calls Only" or "Limited Coverage" when there is no o2 (for sake of argument) signal but there is other signal (rather than "No network" or something for when you really are out in the middle of nowhere) and allow you to dial 999, 112, 911, and possibly some others too.

Two of the three have nothing but a 'dial SOS' button if there's no SIM card in.

All these bizarre links saying that you can't dial 999 without network coverage are, as far as I am aware, incorrect or massively out of date.