some of the Rotts Ive seen in dog shows twenty years ago were a little different than todays. The new lines are a little softer, not hard lined and questionable expression. Your dog has the better trait than the old Rotts, softer in lines top and bottom in profile, large chest and barrel, straight legs in block form, wide stance and the expression is sweet and soft. A very good stop on the face as well! Well balanced all around, not high in rump because some of the old ones, they were a little rounder in hind ends that I was afraid if breeders go that route, they would have the bulldogish type like in QHs.
As for UKC, any breeds of dogs can be shown, including mutts. I have a friend's daughter who showed her Newfie in the last couple of years and the dog was not papered, he came from the pound.