You would expect the Firearms experts at BASC to know what they're talking about.
If both BASC and the Home Office are in agreement perhaps they may just be correct.
Given that they are correct then the first BSA Ultra's (10.5" barrel), Skans, Falcon FN8's, Webley Trackers, to name some, are Sect 5. There will also be a host of air rifles that have had their barrels shortened to possibly below the magic 30cm.
If BASC and the H.O. are correct then why have manufacturers sold Sect 5 rifles ?