Well, we used to call it the WC (water closet), and I've also heard it referred to as the 'bog' (lol, local vernacular possibly)
I found this though:
This British colloquial word for "toilet" was established usage by the 1920s. Suggested origins include:
French _lieu d'aisance_ = "place of easement"
French _On est prie de laisser ce lieu aussi propre qu'on le trouve_ = "Please leave this place as clean as you find it"
French _Gardez l'eau!_ = "Mind the water!" (supposedly said in the days before modern plumbing, when emptying chamber pots from upper-storey windows. According to Chris Malcolm (
[email protected]), this phrase is still sometimes used by common folk in Edinburgh when heaving water or slops, and tour guides say that it originated there circa 1600.)
"louvre" (from the use of slatted screens for a makeshift lavatory)
"bordalou" (an 18th-century ladies' travelling convenience)
"looward" or "leeward" (the sheltered side of a boat)
"lee", a shepherd's shelter made of hurdles
"lieu", as in "time off in lieu", i.e., in place of work done
"lavatory", spoken mincingly
"Lady Louisa Anson" (a 19th-century English noblewoman whose sons took her name-card from her bedroom door and put it on the guest lavatory)
a misreading of room number "100" (supposedly a common European toilet location)
a "water closet"/"Waterloo" joke. (James Joyce's _Ulysses_ (1922) contains the following text: "O yes, _mon loup_. How much cost? Waterloo. water closet.")
http://www.yaelf.com/aueFAQ/mifloo.shtml