Is it? In EU 112 is legally mandated. Did they roll back after Brexit?

Either way the numbers are mostly legacy. On mobiles emergency calls are a special protocol, no number is really transmitted to the network. If the phone application recognizes a certain number it will initiate an emergency call. And quite often several numbers do it. 000, 112, 911, 999. (Admittedly it's many years I last tested it.)

It's always been 999 in the UK. 112 works on phones too, but that wasn't because of the EU: it's part of the GSM standard.

Both are official https://www.gov.uk/guidance/999-and-112-the-uks-national-eme...

112 was made official by EU in 1991 when GSM did not exit yet: https://en.m.wikipedia.org/wiki/112_(emergency_telephone_num...