Hi. Attended AOSB Briefing on Tuesday so think I'll be able to help you out.
Essentially, prior experience matters, but it's useless if you don't perform over the 1.5 days. My leadership experience was extremely limited -- 18 years old and still at sixth form, not a team captain or prefect etc -- but I still pulled off a Cat 1. It's not going to help a huge amount in their decision, but if you apply the skills you've picked up there, it could help you perform better.
Now, come Main Board experience will matter more because the standard is higher. However, my lead recruiter told me quite plainly that you can have all the experience in the world and mess up, or conversely, have no experience and impress. I'd say half of the people in my group at Briefing were in UOTC or cadets, and there's no arguing with the fact that it shows initiative and helps build a profile of someone who wants to go to Sandhurst. As someone else stated, there are routes other than OTC for gaining experience and all of them are equally as good.
As to the benefit of doing UOTC compared to not doing it? From the answers I was given by those in my group, it does make you comfortable in a military atmosphere, which might help calm the nerves. Things like fieldcraft will inevitably help. However, UOTC are explicitly not allowed to coach you to pass AOSB so you'll be doing most of the prep yourself.
Hope this helps.