I wonder if they invite more "maybe cases" to interview if they're from England, because it's less trouble for them to travel to Oxford. If they think someone doesn't really stand a chance, they won't want to drag them in from across the world, but they might be willing to drag them in from Lancashire (my situatiion!), just in case.
I know that Balliol has quite a strong international identity, so Im not sure why they'd have any other prejudice against it. However, I do think that if the tutors don't think your country's education system is as good, or that your exams are a good measure of your potential, they may turn someone down because of it.
It's still unfair though. Maybe the leftyness of the college turns them against anyone who can afford to spend the extra money to study in the UK. Pretty unlikely, I hope; that would be really naughty of them... Je sais pas