Most combined law programs will have very few electives if any at all, you do get to pick some elective law subjects in your fourth and fifth years. If you want to take extra subjects just out of interest and they are within the ambit of your program, then the enrolment system should let you (if not, I suppose you can contact someone and fill in a form to let you do this). Doing extra units for your own interest is fun, and will let you explore more subjects of your interest and would probably mean that you end up at uni for slightly longer, depending on how many extra units you want. (If you still want to finish within the 5 years, you can consider doing them in Session 3/Summer School which lets you do 2 units - so over the course of 5 years and suppose you do 2 units each summer = 10 extra units and still completing within 5 years).
Lots of options available, just have a think about it all