Are they really the same thing, though? If I search Google Images for "washbasin", I get row after row of plumbed-in fixtures that we would generally tag as sink. A search for "washbowl", on the other hand, produces a fair amount of the plastic bowls that comprise the majority of the washbowl tag.
Also, typing the first five letters of washbasin autocompletes to washbowl anyway, so I have doubts that this will make much difference for anyone trying to find the tag.