URD started life as a Poco oriented product -- to run at medium voltages.
The scheme morphed over the years... becoming quite economic for low voltage feeders, too.
(In the Poco world, low voltage means 600VAC or less.)
So what had been strictly a Primary Loop conductor came to be accepted by the Pocos for Secondary Service Taps.
If buried directly in the soil, URD carries ampacities of over 200 in 1/0. The industry makes this with or without a reduced size neutral. For our OP, I would recommend staying with a full sized neutral as he does not have any meaningful 240VAC loads. He has to anticipate that the neutral for this installation will see wildly unbalanced return currents.
[The only time that you can safely get away with a reduced neutral is if you have significant line-to-line loads that won't load the neutral -- ever.]
IIRC, the OP claimed that this feeder would be limited to 100A across 240VAC. So the fact that he's feeding a 200A sub-panel is not a deal killer.
I can't say as I'd recommend it: 125A and 100A single phase panels are common as dust and much more appropriate.
With such intermittent loads, the actual panel size is not indicative of the load to be imposed. Watch out for build out at the new location. I'd certainly leave behind documentation as to the limits of the feeder, etc.
I'd also recommend using only high quality splice techniques in the outdoors for aluminum URD connections.
I tend to favor epoxy encapsulation -- or its equivalent with Polaris and its rivals. That's no junction where you'd want to take a risk.
Be sure to lay down caution tape and a backfill with sand around the URD. Sand is dirt cheap, and easy to compact. It won't bite you. It's astonishing what shifting cables and stones can do to the dialectric.
