I've just sifted through the emergencies section of the UK RT Manual (CAP413, ed. 16) and there is zero mention of the word guard. The word does not exist in that document at all!
The same in Australia and I find it quite annoying. There is no mention of 'guard' in Aus yet the term is used on-air quite a lot.
I refuse to recognise its use and always use/say 121.5.