Say the winner of BGT was against the royal family and refused to perform for the queen, what would happen?

Would 2nd or 3rd perform in front of her, or are republicans automatically disqualified from the semi stage?

I don't think they would audition in the first place if they were anti-royal

Well everyone that enters knows that is half of the prize so they would perform either way..

I wouldn't enter a competition if I didn't like the prize

The winner would be under contract and would have to perform.

Maybe they'd moon the Queen during the national anthem.

They would perform. They just wouldn't gush about the Queen and the Royal Family.

The winner would also be reminded what the concert is all about.

The Royal Variety Show is not a show put on for the personal benefit of the Queen (even if Simon Cowell and the other judges might give that impression). It is a fund-raising event for the Entertainment Artistes' Benevolent Fund (EABF). It is called "Royal" because a member of the royal family always attends and because the Queen is Patron of the EABF.

The work of the EABF
http://www.eabf.org.uk/work-of-the-eabf

History of the Royal Variety show
http://www.eabf.org.uk/royal-variety...-royal-variety

Like others have said. They know the prize before they enter. Plus if they do anything childish they probably lose public support.