Low contact hours don't mean low study hours.
Law subjects have one of the lowest contact hours (3 hours a week until week 6, 4 hours from week 6-12 only if the subject has marked tutorials), but they require the most work.
Most humanities subjects require a lot of outside reading and research to pass. A common misconception is that arts subjects are easy to do well in. Most of them are not.
Keep in mind that bad arts marks will negatively impact on your GPA, which potential employers will be looking at. Law subjects are incredibly hard to do well in - you're going to need your arts marks to push your GPA up, not further down. A credit in a law subject is an excellent result (I'm not making this up, I have heard several lecturers say this).
Top- and middle-tier law firms generally don't even bother with applications from graduates with less than a distinction average.
With that in mind, from personal 1st hand or 2nd experience, history, international studies, politics, English/literature and gender studies all have quite low contact hours (on average, 3-4 hours a week). Sociology is apparently extremely easy to do well in, not sure about contact hours though. Ditto for criminology.