I don’t think it’s meant to be a particularly secret phrase. “Is Angela here?” is quicker and easier to say than “I feel at risk, please can you help me” and a frightened woman would have to gabble out an explanation in a crowded bar. Someone who is genuinely frightened might not easily find the words to ask for help; using a code phrase can feel safer, even if an aggressor might also know it. It also means that in venues that have trained their staff properly, there is then a procedure in place, ie taking someone out the back. Before the campaign, was there a general procedure for what staff should do if a customer says they don’t feel safe?
As for displaying in it men’s toilets: men can feel unsafe in bars too, perhaps from other men. A man might suddenly see someone who has a grudge against them. Asking for Angela provides a discreet way out, and can alert the staff that something is up. A man might have been threatened in the toilets by another man with (for example) “I’m going to smash you for looking at my girlfriend!” I know that sounds silly, but it’s the sort of thing a man who has had too much to drink might say.
And as @ThursdayNext1 says, if the staff say “are you asking for Angela?”, that shows they know the campaign. Someone suggested calling the police on your phone; difficult to do discreetly in a stressful situation. Anyone who has tried to “log it with 101” knows it is not a simple procedure. By asking for Angela, someone instantly has some backup, from the bar staff, and maybe even from other customers. I think this campaign is more valuable than some posters seem to think. It might be touch and go whether it actually works, but it helps everybody knowing that there is a procedure is in place.