It's just a regional or cultural difference. We don't say 'loo' in America. Rarely, someone will say 'toilet.' Usually, we say 'bathroom' or 'restroom.' I knew a British gal who marveled at why we would say 'bathroom' in a public place, because we aren't going there to bathe!

It really depends on the region and the people you talk to! "Toilet" is the standard word, but as Susanne has pointed out, "restroom" or "bathroom" is preferred in the US. "Toilet" might sound a little vulgar there. I normally use "lavatory", which amuses some people. Or "gents"/"ladies". It's clear and polite without being too blunt. Then there are more colloquial words such as "loo" (from French "l'eau"), "lavvy" and "dunny"(Aust). I think you should just use "toilet" until someone corrects you and tells you their preferred word.

I'm off to the loo, off to the toilet, off to the lavvy - a Scottish way of saying lavatory, I'm off to the rest room are all different ways of saying the same thing. In some parts of England they will say 'I'm off to the loo' but also use toilet, bog, and any number of other things. It's all synonyms and localised ways of speaking.