To add to Rob's most recent thread, Home Office Guidelines and circulars cannot and will not ever supersede legislation even if there are errors, loop holes or grey areas in the law. What it does do is open it up to legal challenge should there be a prosecution and even a blanket policy decision not to prosecute by the cps or other legal bodies.
The only thing that can change the law is new legislation, amendment or a stated case and for the latter it would require a case being heard in a criminal court, which is highly unlikely due to the stance of the CPS and ACPO.
It appears from correspondence to do with the legislation that it was never intended to apply to airguns under the legal limit, but it does, are there likely to be any prosecutions for possession of such a rifle, no.
The reason there won't be a prosecution is because the CPS will not support it and neither will the Association of Chief Police Officers; this does not prevent the seizure and destruction of a such gun at this current time.
One thing I think we can all agree on is that this anomaly or whatever you wish to call it needs clarification.