Based off my own experience being in sixth form & chatting to mates, biology is quite tough at A Level [it gets very detailed] so definitely takes a lot more effort & revision than it did at GCSE.
Psychology I heard is super interesting but you've got to memorise facts from case studies to quote in exams, which some students find tough to do.
If you want to do Law, maybe take Psych if you don't already have another essay subject?