I'm an American BA degree candidate at KCL, and although I don't have US undergraduate experience, I was educated for 12 years in the US system, and most of my American friends are in the US higher education system now.
I think that the UK undergrad students are somewhat more focused, but that fits in with electing to read for a particular degree and not having to mess around with the general ed requirements. In London, at least, many kids in the first year are more "adult", even if they live in halls, as IMO, dorm life in the USA is emphatically not independent living- it's like living in a rather expensive youth hotel where everything is laid on for you. You are expected, in London, to shift for yourself after 40 short weeks in halls. There is a big drinking culture in the UK, which I personally don't find particularly appealing, but it's definitely an omnipresent social glue.
In the UK, there are comparatively fewer social events, notably sports, although there are good clubs and societies in the UoL system. I think that, overall, UK university culture is more about the academic work. Kids can and do play around a lot in the first year, but it gets tougher as you progress in the degree. Typically, the weighting of a three year degree is: year 1: 1, year 2: 3, year 3: 5.
I agree with Pendragon- getting a 2.1 is basically equivalent to an undergraduate A grade.
Very interesting topic, zackinbaltimore- do you by any chance go to JHU?