Tallow and polymer pellets
According to the UK news outlet The Sun:  


  The new polymer note uses beef tallow made from suet, which is hard fat found around the animal’s kidneys, stomach and other organs.
  [...]
  The animal product is used as a source of stearic acid.
  This allows the note to be lubricated and to ensure it is smooth inside your wallet.  


Suet is mainly used to make tallow, though it is also a widely used cooking ingredient.  

Stearic acid is a waxy material who's ionic form (stearate salts like sodium stearate) is widely used in detergents. This property gives it the desired smooth, slippery property desired in the manufacture of the polymer notes.