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Just the word itself is a subject of cross-cultural confusion and discomfort. To a Brit, a bathroom is the place where you have a bath. It took me a long time in the US to get comfortable with asking where the bathroom was when I needed to relieve myself. Conversely, my Wife was quite shocked when in England to hear people ask where the toilet was. In polite company, she might hear a British lady talking about going to powder one's nose, but she might also hear someone announce that she was going to spend a penny. The origin of the previous phrase, incidentally, comes from a time when to enter a stall in a public convenience one would need to insert a penny in a slot in the door.
British slang words for the toilet also include loo (a discussion of possible origins of that word can be found here), khazi (it's likely the origins of this word come from the Italian casa, meaning house), bog, lav (short for lavatory, another common term) and WC, short for Water Closet. British friends visiting America may hear slang words such as head (this is thought to be due to the location of the crew toilet in the bow or head of a ship) or john.
I cannot let this talk about bathrooms go without a little explanation about plumbing, for the benefit of any transatlantic travelers out there. When I first encountered an American toilet, I found it somewhat scary! I thought it was blocked and about to overflow. I have since learned that American toilets have a higher water level than the toilets I was brought up using. Conversely, American travelers to Britain should be aware that yes, British toilets are meant to have a low water level. The water has not drained away. This difference means that British men using flush toilets (as opposed to urinals) in America are forced to subject anyone within earshot to what I refer to as "The Niagara Falls Effect," whereas in Britain, some careful aiming results in a discreet, almost noise-free performance.
On the subject of urinals (sorry Ladies), in the restrooms (I am certainly not going there to rest!) of pubs and other public places in Britain, you will often find a communal along-the-wall urinal, rather than the individual units that are more common in the US.
Then, of course, there is the infamous British love of toilet-humor. More on that later, I think. I need to go wash my hands.
I am a little cautious about opening this one up for public comments and discussion, but please do let us know your thoughts on this subject.