I would argue that we misinterprete this on a regular basis, and that just having a UKARA number does not automatically mean one can legally buy a RIF. The exceptions on RIF ownership given to airsofters is so that we can use RIF while skirmishing, you need to be an airsofter to buy a RIF. The intention being to enable airsoft games to be played with RIFs.
Now the usual advice is generally 3 months + 3 games minimum (or whatever the exact wording is) then get UKARA number. However, if your only intention with the games are to acquire a UKARA number and you stop going to games after you have acquired it you can not legally buy a RIF, as you are no longer play airsoft.
As always, I'm not responsible for your actions, read the law for yourself and make your own mind up. Opinions?