I do question their screening process on BGT...
Hahahaha! Screening process? I'd imagine the screening process for most reality shows goes like this:
Stage 1: Do you want to be on television?
Stage 2: When you are on television, do the ratings go up?
If you answer yes during Stage 1, congratulations, you're on television. If after you've been on television you can answer yes during Stage 2, congratulations, you're going to stay on television!
After the massive hype following her first performance, the BGT producers would've wheeled Susan Boyle on stage strapped to a dolly (hand truck) if that's what was needed to get her back on camera.