Toilets have a nifty history. For instance why are they sometimes referred to as "JOHNS"?

The  most common, semi-euphemistic slang term used in England now, is probably "loo".

The word "john" is very rare in England, nowadays anyway, whilst  "potty" is short for " chamber pot" and generally now means the one used by very young children. The type used by adults is the "commode".

When few homes were lucky enough to have the lavatory within the building, most people satisfied nocturnal needs with a chamber-pot that in some parts of England at least, was called the "guz-under", 'cos it "guz [goes] under the bed". My Grandad and Grandma had their loo in a brick outhouse at the end of the back yard - it was a flushing one! T'other grandparents' home were more up-market with the loo just inside the back door.

Sailors, at least in the Royal Nay, call it the "heads", which might refer to the location of the seamen's toilets on 18C sailing ships often being in the bows - the head of the ship. An old British Army slang word was "khazi" (pron. as in "khaki") and this was lampooned in the 1960s comedy film Carry On Up The Khyber, in which the British colony's ruler's title  was "The Khazi". Khyber, from the Khyber Pass (a real place), forming English rhyming slang for what in English is indelicately called the "arse".  

You also hear euphemisms like "smallest room" and "bathroom" - the later perhaps because many homes do have the w.c. in the same room as the bath.

While the act of using the place has all manner of euphemisms:

 - "spend a penny", from the first public toilets having one-penny coin-operated locks to help pay for the provision and maintenance.

 - "leave the room"

 - "answer a call of Nature" 

 - "go to powder my nose". That's rare nowadays.

 - "relieve myself"

 - "inspect the facilities"

 - "take a natural break" or "comfort break". These are usually used in formal meetings.

 - "get up in the night" is a phrase that chimes particularly with males of wise and even wiser, age.  

 - "stretch my legs" rather depends on context and is often used almost literally, of going for a short walk such as to relieve the aches and pains of sitting in a moving vehicle or uncomfortable room for a long time. Its origins might be in the early railway carriages having compartments with not connecting corridor, so passengers had to sit with their legs crossed until the next station where, with luck, they had a couple of minutes in which to relieve themselves in facilities in the station building.    

Railway stations built in the 19C in Britain had a Gentlemen's lavatory, just signed "Gentlemen" or "Gents", often placed outside of the main building; while within the building was the "Ladies' Waiting Room". It was indeed a waiting-room for the fairer sex only, but it also disguised the location of their necessary facilities - reached through a door from within that room.

"Toilet" originally meant washing, as on getting up in the morning, and was still used to mean that in the 19C.