Rewriting the law based on a police rewriting of history is something which seems to happen all the time - and the 'tough on crime' legislators welcome it. Beery's reason (Lee Rigby) for the obsolete calibres thing wasn't something I was aware of - but sums it up perfectly. In a similar case we have top cops using Dunblane as an excuse to arm officers here - even though armed police wouldn't have prevented it happening.
One suspects that in reality the police would like to ban all deactivated weapons, obsolete calibres and firearms - and there are plenty of rentaquote MPs who would entirely agree. Each law creep should probably be seen in this context.