British electronic singer-songwriter Katy B and her new collaborator Craig David appeared on BBC Radio 1's Live Lounge today, performing their single "Who Am I," as well as a surprise mash-up cover of Sigala's "Say You Do" and Mariah Carey's classic "Always Be My Baby." Playing off the similarities in the songs' choruses, Katy B's version is a slow, piano-driven ballad that…