Just to relieve you worries - it's not an interview. You will literally be handing your documents to someone instead of putting them in a mail slot. It's just a faster processing time. As a member here once said, it's like a fast pass at Disney World. You ride the same ride, just one person gets to cut the line.
They won't expect you to have your name on 6 items of mail (that is, after all, the requirement after 2 full years in the UK). If you have your name on anything, bring it with you. The two items you have will be fine. I'm surprised people are telling you that you need to be married first though.