She wouldn't be dumb enough to NOT perform 45S when the video just came out and the song is still gaining traction across the globe. IF she decided to perform another song I am almost positive it would be a medley of 45S and the second single OR one of her more recent songs(Stay, Diamonds), hell even an album track from Unapologetic.

She isn't performing at the Brits, like one user already said they have filled their performing slots.

Well she IS going to be in London and
I'm sure producers would be able to fit one of the UK biggest star in their lineup if needed.

Also they had 9 performers in 2012.