I think so - that's what I've heard, and fits with my experience (though I haven't had my offer yet).
I went to a modern languages one last week having been recommended for offers for both of my applications (I applied twice). There was a general talk from the admissions tutor about how great UCL is, then a couple of mini-lectures about language related things, then we each had an individual timetable. Most people had a chat for each language and a tour for one of them but I had 3 chats as I had 3 languages. We just sat around tables with a couple of other students, some teachers and sometimes some current students, and we talked about what we wanted to do and asked questions about the course. It definitely wasn't an interview, though - the only time I was asked anything remotely interview-ish was in my discussion for Scandinavian languages, where they asked what languages we'd chosen and why as they're not ones people study at A-level.
We were explicitly told that we would be receiving offers, and to expect them within about 10 days. I haven't heard yet but it's only a week today so that's hardly surprising.
I think it mostly seems to be a test of commitment - for example, I know some people who have already had offers from their favourite and somewhere they'd rather go than UCL, so they just didn't bother with the open day as they knew they weren't going to take the offer anyway.