The 112 number has been introduced as an EU-wide common number. Because of traditional use of 999 in the UK, this number will be retained. The problem with 112 is that it is very easy for noise on a line, or a customer tapping the phone cradle, to result in a false call. (That's one of several reasons why dialling 999 was originally chosen).
When you dial, the operator sees your number through a Call Line Identity system. (This will always happen, whether or not you have dialled 141 first, or have arranged that your phone number remain hidden). Using a daily updated data base, the operator can then find the address of the line installation, with the postcode, the name of the phone customer, and the service provider. This information allows the nearest emergency service to be selected.
With mobile phones, caller location can be immediately pinpointed and is available to the emergency services through the Enhanced Information Service for Emergency Calls, the result of collaboration between BT, 02,Orange, Vodafone and 3.