Right, this is how it works.
1) Does the defendant accept that the object in question (ie a knife, axe, machete) was found in his possession outside of his home or another private place / place of work?
If yes, then:
2) To be acquitted at trial, the defendant must demonstrate that he had the object with a lawful excuse ie the defendant is a lumberjack and was on his way to Sherwood Forest and that is why he had the axe in his hand. This 'defence' must be demonstrated to the Civil law standard of proof which is that the tribunal must accept his explanation proven on the 'balance of probablities' - in ordinary language, that the defendant's excuse is more likely than not.
THEREFORE, once possession of a prohibited object is accepted, the burden does indeed shift to the defendant to demonstrate lawful excuse, albeit on the lower standard of proof, not 'beyond reasonable doubt' which as everyone knows is the criminal burden of proof.
I hope that's clear. Rather than checking the CPS guidance, check out the law itself or even wiki crime, which is quite good (though not as good as Archbold which i note you have quoted above)
http://en.wikipedia.org/wiki/Prevention ... e_Act_1953