I don't know whether this has been covered or whether you already had your meeting. But... my 2 cents.
Get all the documentation you can from independent sources. You don't want to print out anything from an apostate website. Get it from the UN's site or whatever. The Guardian article. NOTHING can link you to apostate sites.
THEN, get some prime quotes about the UN from the Watchtower. Maybe a half dozen quotes or so all printed showing how evil the watchtower believes the UN to be.
Then ask how they can be involved with the UN in any way shape or form? If questioned about where you found out, you can always say that some householder attacked you with it. They don't know I'm a former JW here and I've brought that up to them sometimes. Yes, the guy at the door could be apostate, but you're just doing your duty of going door to door. What? Should we no longer talk to people at the door?
You didn't believe it, but he was so convincing it was true and urged you to look it up on the UN's site, so you did.
The standard defense is that they just had a minor type of membership to get access to their library. If you can find rules on the UN's website, you could show them that library access is available without a membership. (although that approach assumes you already know what they will say - maybe scratch that).
You could go further and find out the requirements of the UN membership where they have to promote the UN. You could mention that surely the society wouldn't do that, but then you started to see articles here and there that were favorable to the UN - this villain of Satan's - and you were shocked and stunned. And you didn't know what to do, so you froze. You were shell-shocked and didn't know which way to turn. Print those articles as well.
I agree with others. They probably won't care. They want to know where you stand now. Are you willing to forget about this, jump back in line, and not spread this info to anybody else? If they can be convinced you're shell-shocked and need their help, they may just welcome you back. But they will keep an eye on you from now on and the first thing that goes wrong, you're out.
And, by presenting all of this evidence from non-apostate sites, you might plant a seed with one or two of the elders as well.