What is Sonakshi Sinha up to in London?

Sonakshi Sinha has taken off to London for a break. The actress posted a snap on Instagram, with the caption, "Shakespeare knew what he was talking about."

The pic has her posing in front of a store with the line from The Tempest, 'Hell is empty, and all the devils are here' written across the store window. Who the devil is Sona referring to?