I see you're a 1K, so you'd get access as a *G anyway. Of course, if you're not on a Star carrier flight you could hypothetically use your S+ card for UC access. The agents will very likely not be familiar, so ask them to swipe the card and that should bring up the relevant details on their screen. Otherwise you may need to ask them to look it up.
But in theory, yes, as long as it's an actual, United-run UC (not a Star or other partner lounge), you should have access.