PU is short for polyurethane resin. It usually comes in white or tan color (although it is usually translucent until it sets), but you can also pigment it to any colour you like. You can get lots of different types of this resin with different properties (hard, soft, rubbery etc.). The rudder and hydrovanes shown In Trevors build are cast from PU resin.
Good UK suppliers are Alec Tiranti (google them), and if you can find a stockist Smooth-on do an extensive range range of this type of resin.