As with many English words, some are common in American English and others are common in British English. However, words such as: bathroom, ladies room, men’s room and restroom are common to both. On most airlines, the toilet is referred to as “the lavatory”. You might hear the flight attendant say: “The lavatory is located at the back of the plane“.

In a formal setting, it would be polite to ask: “Excuse me, where can I find the ladies room?” or “Excuse me, could you show me where the restroom is?” or if you are at someone’s home: “Sorry, where’s the bathroom?“ 

When you are with friends the setting is usually more informal. In these cases you can use words such as: the john (men), the can, the loo (ladies) and TP which means toilet paper. 

Tom: Hey, Sam. Where’s the john? 
Sam: It’s down the hall. 

Mary: Hi Sam. Where’s Tom? 
Sam: Oh, he’s in the can. 

Instead of telling everyone that she needs the toilet, a lady can say: “Excuse me, while I go powder my nose“. 

Of course, the most basic way to ask is: “Where’s the toilet?” However, there are many words for the toilet so choose the appropriate word according to your listener.