For a Law degree it doesn't matter which A levels you are doing - you just need to be confident of getting AAA and above - so pick a subject where you feel confident of getting that.
For History this might depend on the syllabus - ie. the periods of history covered - and how much this interests you. Sociology is not a 'soft' subject at all - it is a solid essay-based subject and regraded equally to History in terms of University admissions. You might find this useful for an understanding of contemporary society, and current issues. Talk to relevant teachers, and ask to have a look/read of relevant text books or revision notes etc.