Shouldn't "bathroom" and "restroom" be also accepted?

In England we don't usually talk about restrooms, and bathroom usually means a place where you have a bath - it may or may not have a lavatory.

Is there a parallel form with -szoba?

But the asking person was meaning a washroom , isn't it clear ? Don't argue!..

Where is the sink?

Lavatory is commonly used in UK & Ireland for bathroom/toilet