In that case, it will again not benefit from the 1988 Act because the item specifically say that it does have a proof House Certificate.
Next, European Directives are not Law. That directive will have had to rolled into English Law somewhere or, in my opinion, one will be relying on the European Court of Justice. Additionally, in terms of Firearms Laws, Member States are allowed to have stricter firearms laws than the EU Directives if they so wish. For example, we are not allowed to own pistols whereas the rest of the EU is.
Finally although the 2006 Violent Crime Reduction Act specifically excludes from the definition of a “realistic imitation firearm” anything that: "is neither a de-activated firearm nor itself an antique" it does not make it clear whether or not the "de-activated firearm" MUST be one within the meaning of the 1988 Act. In other words this could be another one for the courts.
BTW I would be interested where the ownership of a de-activated Firearm by an under 18 is prohibited by English Law.
Finally I wonder where on earth they find people with fourteen hundred quid to spend on one.