As above Donzie, the VCRA is the relevant piece of legislation here, not UKARA (which is just a database). Weirdly, as long as you're over 18 it's never illegal for you to buy a RIF, so you're covered either way.
For your friend to have the defence he'd probably like to have as a seller he needs to feel like he's met the demands of the VCRA, which is that he believes that he's selling the RIF to you for "the acting out of military or law enforcement scenarios for the purpose of recreation" and that this will take place at a site with the relevant third party liability insurance. Under that definition you don't even have had to have played airsoft yet for him to sell you a RIF, but since he's played airsoft with you previously and recently then he's covered.
Long story short: he can sell you a RIF.