Wax was a malleable substance.
Madame Tussauds, known for its wax figures, commissioned Auto Plastics Ltd. to produce new figures for the museum's tableau of senior British government civil servants and military officers made out of plastic instead. (TV: Spearhead from Space)
When Charlie Sato first became a warrant officer for the Vault in 2011, he was given a tour by Captain Ruth Matheson. He saw four objects connected to the Doctor: An army jacket, a painting, crystal and a wax cylinder, which bore a significance. Each was used in one of the Doctor's many adventures. (AUDIO: Tales from the Vault)
Behind the scenes
Multiple Doctor Who related wax figures were constructed by Madame Tussauds for the original Doctor Who Experience. One of these was infamously used in promotional images for The Five Doctors. (DWM 48)