That's really interesting that you have that view. I did a science degree at a Russell Group University (but not Oxbridge) and it was a standing joke there that humanities subjects were easy and required little time commitment. I always used to argue with people who said that because I thought it was a very patronizing attitude, but obviously I've never done a humanities degree so I don't know.
In science, we had a lot of classes (frequently 3 hours of lectures and 3 hour long lab every day) and a lot of homework, and we were hard at it all the time. I found the step up from school to University very significant. I know that in our place the first year was set very hard because they wanted to drive out any students early on, who wouldn't be able to manage the whole degree course, rather than have them limp through several years and then fail.
In the dentistry course I know that in the first day they put a severed human head on the bench at the front so that they would scare off any kids who weren't tough enough to make it through the whole course. I'm glad I didn't do dentistry.
Like Em I know there were subjects I couldn't have done, for example maths. I just would have found it too dry. And I didn't do physics because I used to get bored at physics class in school and end up cramming the whole year in the last week. I don't think that would have worked well at University. 
Wreck it really sounds to me as though some courses are very different from others and that a lot of it depends on how much you yourself are interested in the course. Perhaps it would be worth talking to people have done the specific course that you are interested in?