If you were registered with one of our sites and asked us in advance we would happily provide confirmation of your current UKARA status to a third party. As far as we're concerned, it's part of the service paid for by the UKARA admin fee we charge.
Don't forget, UKARA was put in place to prevent retailers from going out of business, from a business point of view I can understand if a retailer is reluctant to come forward with details that help you buy a RIF from someone else, it's not how the process was intended to be used and they're opening themselves up to being dragged through court to defend somebody else's sale...
If you haven't done so already, contact the site operator who your membership is with, I'm sure they will be more than happy to help.
On a related note, I do get frustrated when I get calls or emails from sellers saying "I've got Fred here, says he plays on your site and wants to buy a RIF, can you confirm him?" Can you please ring me first and ask so I've got half a chance of figuring out who you are? My memory is shocking especially with names, and I may have 3 or 4 Freds playing on different sites.