I don't know much about the UKC registry, so take my opinion for what it's worth.
My boxer Charlie had an AKC ILP, which was like a limited registration (not exactly, but similar). I didn't have a regular registration on him because his parents only had the litter registration (apparently, the breeder demanded proof of spay/neuter before allowing registration, or something like that).
It allowed me to show him at obedience matches, rally, and stuff like that. He was ineligible for conformation shows.
All I had to do was provide photos (front/side/rear), provide proof of neutering, fill out the paperwork, and pay the fee.
I thought it was worth it, because he was not breeding material anyway (wry mouth, among other things), but I did want to show obedience with him so it served my purpose. It was not expensive, IIRC.
Hope this helps.