That's all true, I just don't see why it's necessary for the author to have a snotty attitude when asked. (And to be clear, I was talking about JKR's attitude, not the actresses)
Also, when you're writing an urban fantasy novel series with worldwide scope, it's not unusual to visit other countries. Harry Dresden of the Dresden Files (a comparable series) visits Scotland all the time even though the series is set firmly in Chicago and is written by an American.
As far as why they're asking the actress, I think reporters want to think outside the box. I was a PA once George Clooney was shooting a movie here in North Carolina. They had a media day where reporters from all the other world came in and interviewed Clooney all afternoon. Each reporter had 15 minutes to interview Clooney. I stood there the whole time and I heard some questions 500 fucking times. I don't know how Clooney didn't just tear his head out. But he was patient and answered every one like it was the first time it was asked. But he lit up whenever a reporter would ask an oddball, off the wall question. See, some of the reporters were savvy enough to know all this, so they start off with a couple oddball questions to break the actor out of the monotony and get them paying attention to the interview.
I'm sure this is behind almost all the times you've heard a celebrity blurb ( "Brad Pitt says he always wanted to be a sauce chef" ) that just seems so off the wall and makes you wonder how the hell they got on that topic. So just a guess, but I suspect that's how the Hermione actress could get on that subject.