The rules of the VCR Act always apply. But UKARA itself is merely a mechanism to facilitate selling and mailing stuff to buyers and remaining on the right side of the law: it allows retailers to easily check up on whether a customer is actually an airsoft player, without having met that person and checked their reasons for wanting an RIF or had someone vouch for them in person; and it provides a convenient way for buyers to prove that is the case so they can get stuff mailed to them rather than having to go to great lengths to prove their legitimacy for wanting an RIF.
But ultimately, both those functions of UKARA registration are in place so that a retailer can conduct their business and can provide a defence if an attempt is made to prosecute them for having sold an RIF to someone who subsequently uses it for nefarious purposes. Nevertheless, if a retailer or other seller has a good reason to believe that the person buying an RIF is doing so for a legitimate purpose, then that is enough of a defence for the retailer, without them or their customer having anything to do with UKARA at all. For example, I work in a job where I do movie special effects, so there is a legitimate reason for me to own RIFs, regardless of whether I do airsoft skirmishing or not. If I did not do skirmishing, but proved that I worked for the movie post-production company that I do work for, then a retailer will cheerfully sell me an RIF, as they can cover their ass in the event of an attempt to prosecute them if I subsequently flip my lid and hold up a bank with the thing. Of course it is simply easier for me to to be UKARA registered if I want to buy RIFs mail order, but if I was prepared to go through the rigmarole of explaining that I work for the company I do work for, then that would be a suitable defence for them having sold and mailed an RIF to me.
Similarly, and as per the original post question, UKARA has nothing to do with the legitimacy of someone owning or using a RIF. Someone could tip up at an airsoft site with the most realistic looking RIF ever, and the mere fact that they have turned up with that thing with the express intention to play airsoft with it, is in itself a legitimate reason for having the thing. Thus within the boundaries of the rules of the VCR Act.
