Holly, 14, won the Disney Channel's TV talent competition - which was inspired by the movie Camp Rock - after facing a judging panel which included Craig David and vocal coach Zoe Tyler, then winning over the channel's viewers.
Camp Rock star Demi said: "I'm excited to be back in London and it has been an honour to present Holly Hull with her prize."
The US actress - who also performed her new single La La Land as part of the show - continued: "Holly showed she has what it takes to rock the stage but all the acts are extremely talented and everyone should feel very proud of their achievements."
Holly will now go on to release her version of the Camp Rock track, This Is Me, on iTunes on Monday May 11 and create her own music video, which will broadcast to millions on Disney Channel on Friday May 1 at 5.55pm.
She said: "I can't believe that thousands of Disney Channel viewers voted me as the winner of My Camp Rock. Meeting Demi Lovato was the best experience ever and made the Camp Rock dream a reality. I can't wait to watch my very own music video on the channel. I've enjoyed every minute and feel incredibly fortunate."