Problem is the drafters of that particular piece of legislation failed to cater for the fact that airguns are deemed as firearms under UK law, just the ones that are not deemed "specially dangerous" i.e. Under the legal limits, don't fall into sections 1 or 5.
So an airgun that happens to look exactly like a "real firearm" cannot by definition be classified as an imitation
firearm because it already is
a firearm. Oops!
This is why the .177 CO2 pistols that look exactly like 1911s and Berettas can be sold as airguns and not as airsoft RIFs requiring specific defences such as being a reenactor or airsoft skirmisher.