In general, in most colleges, I'd say there isn't a massive amount of mixing (outside of the sports clubs)
However, if you wanted to it would be easy enough - undergrads and postgrads eat in the same place, and mostly live on site, and as alluded to before, mix quite a bit in the sports clubs (especially rowing)
You would have to take the initiative though because most of the freshers events are pretty much just for undergrads (and I guess there are equilavent events for postgrads)