1. Gas guns.
3. It's not even a membership, its a database maintained by a retail association in order to prove to the homeoffice et al, that all the Realistic Imitation Firearms they've sold were to people who had, subject to a prior consultation with the govt, proven that they were airsoft skirmishers/reinactors to a degree that is believed to satisfy the defences in the VCRA2006 to the sale, import and manufacturing of Realistic Imitation Firearms.
It protects the retailers (and everyone else), from breaking that law at the point of sale of any Realistic Imitation Firearms. It is also needed for import and converting a [two-tone] into a RIF without breaking the law.
It's not even like a license because when it comes down to it UKARA is just a way to prove you (until it's tested in court) have a defence to the VCRA2006, it is not a requirement* for importing/manufacturing/selling, but it's the most popular and easiest to work with, and the most expediant if you're shipping them into the UK as a private individual.
*Note, you do require a defence, but it does not have to be UKARA.