The 0.303 Bren gun used bulleted blank. The wooden bullet was necessary to cause the build up pressure in the barrel to cycle the self loading mechanism. A flash hider at the end of the barrel was cone shaped with a semi circular plate at the end to smash the wooden bullet. Bulleted blank was quite dangerous if mistaken for rifle blank, not only for the "target" but it sometimes blocked the rifle barrel and caused a bolt blow back.
The introduction of the 7.65mm SLR required a normal blank that would feed in an automatic rifle . Thus it would feed happily in the GPMG and also the converted 7.65mm Bren gun. A blank firing attachment was needed to cause a build up of pressure in the rifle/gun barrel that could cycle the self loading mechanism. On the GPMG this was accomplished by using a special barrel.
I suspect the wood was sterilised to prevent the blank deteriorating in storage, not to protect the environment. After all a quicker rotting down is better.
As an aside, the "blank" used for mine layer training was filled with fertiliser so that farmers would allow training on their land.