One of the things no-one seems to have mentioned is that in order to get most oxbridge degrees you have to satisfy residential conditions as well as the academic ones. Essentially you HAVE to stay in cambridge during term time, and apply for an "exeat" (permission to leave) if you want to go away for the weekend. Which is really causing issues for one of my friends who is at cambridge and the inability to leave for weekends is destroying his relationship with a girl at another uni.
Some colleges are also apparently very strict about visitors staying over.