I wish I could remember exactly, but my S was in the London program and I think it did have a 3.5 gpa requirement. He's on spring break, but I'll check when he returns.
The 3.5 requirement is for the Donnelley Fellowship, which provides financial support for a year at Cambridge. There is no GPA requirement for participating. However, the competition for the spots at the study abroad sites can be "fierce" as they put it. http://study-abroad.uchicago.edu/programs/british.html