As I see it, Brian, they probably are, under that clause. However, it has been stated and confirmed by the HO, ACPO and the CPS that this wasn't the intention of the law, the HO have explicitly said a sub 12 fpe air rifle is just that, regardless of single shot, magazine or semi auto, when they refer to air rifles they mean that to include all sub 12, and that no prosecutions will follow.
The problem is that their desire to tidy up the law at the first possible opportunity hasn't happened in 17 years or so. Which shows how pressing a need that was.
Why the GTA should choose right now to raise the issue, and it was the GTA taking it up, not the HO being concerned, is anyone's guess.