The distinction here is who is committing the offence: an under 18 buying an airsoft gun is committing an offence, so is the seller. An over 18 buying an airsoft gun is NOT committing an offence, however the seller is if it's a RIF; they may have a defence against prosecution for having committed that offence e.g. the buyer is a skirmisher.
As a result, it's 100% legal for Big Jim (he's 18 or over) can LEGALLY buy an airsoft gun (RIF or IF, makes ZERO difference), whether he can provide a defence to the seller or not he is breaking no law. the SELLER on the other hand cannot sell a RIF to Big Jim unless the seller can prove that they are able to provide themselves with a defence against prosecution for SELLING a RIF to someone. Big Jim has broken no laws at any point.
That's why UKARA exists, to provide the RETAILER with a defence against prosecution, you as the purchaser do not require one as you're not breaking any law.