We call it a loo, or khazi, or bog, or pisser, or shithouse or toilet (does that word make American's blush?) because that's what it is. Or we say we're going to spend a penny, or see a man about a dog.
Its not a bathroom because of the simple fact that there's no bloody bath in there! Or do you lot piss in the bath?!? Or washroom as you also call it. I'm not going for a wash, I'm going to throttle a mars bar!
Not being a British person I feel that I can't really answer questions on this thread but I do follow it daily!
My favourite Irish term for having a pee is "I must go and drain the eejit"