As above, it's not a licence. UKARA stands for United Kingdom Airsoft Retailers Association and is merely a registration system that allows retailers to check that you have a valid defence against the Violent Crime Reduction Act (VCRA).
As long as you are over 18 it is perfectly legal to OWN a RIF. Period.
If you were sold one without requiring proof of defence (UKARA registration or site membership for example) that's the illegal bit. The VCRA makes it illegal to sell, import or manufacture a RIF. The second hand market is somewhat more lax in application of the rules but there you go.
In terms of rocking up to site with a RIF and no UKARA - it's not an issue. At all. Just don't wave it about in public and you'll be fine.
No doubt this thread will now descend into pages of back and forth detailing the whys and wherefores but feel free to ignore all that, it's going to happen regardless and will cease to be relevant to your question within 5 posts.