s a r a - A bathroom is no more appropriate because it usually doesn't have a bath in it.
A 'restroom', well I've never understood why Americans call it that because you don't go in there to rest (you'll get piles if you rest on the toilet regularly for any length of time!).
"Being specific, 'water closet' refers to the room in which a loo is found, whilst 'lavatory' actually means a wash basin (from the latin lavare) and 'toilet' means the appearance (as in toilet water, toilet bag etc.). So there's plenty of room for pedantry in the smallest room.
Here are some of the miriad of terms used to mean the WC, or the room in which it is found:
The ablutions, bathroom, board, bog, can, cloakroom, cloaks, close stool, closet, commode, (public) convenience, crapper, donek, dunny, EC, facilities, garderobe, gents, gozunder, heads, jakes, jap, jericho, jerry, john, khazi, ladies, latrine, lats, lavatory, lav, loo, necessary, netty [from the Italian gabinetto], No. 100, place of easement, the plumbing, po, porcelain pony, powder room, privy, proverbial (brick outhouse), restroom [america only], retiring room, s t-house, shot tower, smallest room, throne, thunder-box, toilet, usual offices, washroom, water closet, WC and you-know-where.
And here are some phrases indicating the actions therein:
answer a call of nature, be excused, commune with nature, do one's business, drop Hitler a line, ease oneself, examine the plumbing, jimmy riddle, pay a call/visit, pluck a daisy, point percy at the porcelain, powder one's nose, relieve oneself, see a man about a dog, see the geography of the house, shake hands with a friend, shed a tear, slash, strain the greens, spend a penny, talk into the big white telephone, tiddle, tom tit, wash one's hands, wring a kidney."