Although perhaps not the mayor of London. As noted about the pic:

Pretty much every American news outlet is gleeful about the “giant Trump baby balloon,” as it is conventionally described, that supposedly is dominating the London landscape while Trump is in England. London’s Mayor, Sadiq Khan, gave activists permission to fly the blimp (as it is also referred to), which apparently was needed because it is so “giant.”

Via SDA for the poll and Powerline for the picture and the accompanying text.