I read Laws and got a 1st. Honestly, it's not the hardest degree in the world - especially compared to engineering and stuff, and you're mad if you don't make time to do other things -especially in 1st year. I wouldn't worry about doing any preparation before the course, there really is no need.
You should easily have time to do a sport, another society, have friends and still do really well. Just don't go over the top - learn the key concepts and then if you really want to devote extra time, spend it reading and understanding cases. It's the sort of course where understanding the core principles of each module and knowing some cases is worth 10x more than a million flashcards and lots of highlighters.