I've done this recently myself. What you need to do is email 999testcalls(at)bt.com and arrange a window to place a series of test calls to both 999 and 112. They will give you a time, a date and most importantly a script to follow (and you must follow it). If you pre-schedule them it's totally 100% legal and supported, if you just place a test call without doing this it is illegal.
Whole thing takes about a day to setup and the BT people are quite helpful once you reach the right people.
https://www.reddit.com/r/sysadmin/comme ... emergency/