It's no problem at all that you haven't done law at A-level if you want to do it at university, any subjects will be fine (humanities and social sciences as you have studied are ideal though because they are essay-based). You could always look into combined honours programmes in law and psychology, there are several universities that offer this. However, make sure you study a degree that is BPS accredited if you want to do further training in psychology to become a clinical/educational/forensic/occupational etc. psychologist. This is crucial unless you want to spend an extra year doing a conversion course. If you want to pursue law as a career, you'll need to ensure the degree programme allows you to continue on to the LPC/BPTC depending on whether you want to be a solicitor or barrister (as far as I know, it depends on the weighting of the subjects which subject is accredited, e.g. Law with Psychology for a qualifying law degree, or Psychology with Law for a BPS accredited psychology degree, but it's important to verify this with each course you apply for).
If it helps I was in the exact same position as you, unsure whether or not to do law or psychology but I went for psychology in the end because I felt more suited to it and I haven't regretted this decision. There's always conversion courses for both subjects if ever you decide to change over to law or psychology after graduating, so you certainly aren't restricted once you've made your choice.