As an American who goes to the UK a couple times a year, I have found that most Brits know exactly what Americans or Canadians mean when they say restroom. Traditionally, in the States, using the word toilet tends to be considered either slightly crude, (as opposed to rude,) or low class. Furthermore, restrooms are always in public places whereas bathrooms or powder rooms are in private homes. Obviously some people use the word bathroom in public places as well, but one would never use the word restroom in a private home. (At least according to Miss Manners.)
Some guys will jokingly use the term "little boy's room."
Being bicultural, I use whatever term is appropriate where I am at the time for this as well as for other terms like lift/elevator.
By the way... I have seen some nice "cottages" in the UK.