Sink is the most common usage in the UK.
if you call it a sink it could be
a bathroom sink or a kitchen sink
but wash basin is just that and as soon as you say wash basin you know what sort they are talking about but yes we in new zealand use sink alot too but its more said in a full sentence
like we need a new bathroom sink or go wash your hands in the bathroom sink or if you are inthe bathroom you might say look at that sink. now if you talked to someone and said i blocked my sink they wont know what sink you mean but if you say i blocked my wash basin they know its the sick in the bathroom lol so its fine how it is