999 is not in the GSM Phone standard (Ref Wikipedia) and so doesn't have to allow access past the phone security. I certainly had a phone (3 phones back) that didn't let you dial 999 without first unlocking the phone but did allow 112.
112 - The emergency number in the United Kingdom
says they can locate your phone within 2 seconds (I guess thats to the mast its using or that it doesn't refer to mobiles at all).
The text service is country specific.
112 has a potential advantage if you are near a border and are actually connecting to the neighbouring country's mast. Guess it may be possible on the odd occassion near dover as you can sometimes pick up french coasties on CH16 in the solent if the weather is right. I guess if you like in the CIs you'd be better dialing 112 in case a french mast got you.