According to City of Westminster archives the sculptor is unknown.
The plaques show (above) the Dome of Discovery and the Skylon, with a clutter of navigational paraphenalia; (middle) the Festival plaque by Abram Games with the date; and (below) the Festival Hall, the old shot tower with the curious 'radio beacon' that was put on top for the duration and a clutter of musical impedimenta.
I remember my father pointing out the shot tower as we walked over Hungerford Bridge on our way to a Robert Mayer Childrens' Concert about 1960, raging at the announcement it was to be demolished. It came down in 1961 to make way for the Queen Elizabeth Hall.