Let's all rewind a second. The 'Airsoft Exemption' states that a RIF may be sold to a person over the age of 18 for the purposes of organised skirmishes at an insured game site.
Nowhere does it specifically state that you must be able to prove you've already done it. UKARA requirements are a belt and braces approach that near enough 100% protects retailers from any kind of legal issue with a sale, nothing more.
To the OP, the Nuprol Delta series are pretty new to the market, though I'd be very surprised if they aren't just rebrands from a big box manufacturer like JG or CYMA. That appears to be the MO for Nuprol.