How should one address a police officer in English speaking countries? More specifically, in a non-emergency situation—asking directions for example—what is the expected form of address used to call a police officer's attention?
I've lived in Britain all my life, and I don't think I would use any particular word to address a policeman. It's seldom necessary to use an appellation in a typical British conversation. Indeed, it would seem odd.
In the US, people seem to like calling each other "Sir" and "Ma'am" as a mark of respect, and I see no reason why that wouldn't be appropriate with a police officer.
If you must use an occupation-specific appellation, then you can use the person's rank. But that requires you to recognise their badges, or you risk getting the rank wrong. AS @Gnawme observes, "officer" is always OK.
Any policeman worth their salt, of course, will treat you politely whatever you call them (unless you abuse them).