Re: "no requirement of 'registration' for deactivated firearms"
It makes sense if you're dealing with firearms which are merely "deactivated", ie barely anything done to prevent them firing, and easily reversed by criminals.
There's much less need for it in the UK because our deactivation standards are much more stringent. Arguably pushing those stronger standards to the rest of Europe would have made more sense, but I doubt the EU is interested in any bright ideas coming from the UK at the moment.