You are back in London or you to London?

The correct sentence is: "You are back in London."

"Back in London" is a prepositional phrase that means "in London again after having been away." It is used to describe someone who has returned to London after a period of time away.

"You to London" is not a correct sentence because it is missing a verb. A verb is a word that describes an action, state, or occurrence. In this case, the verb is "are."

Therefore, the correct sentence is "You are back in London."