Economics, criminology or politicsWatch
If you like Maths for a degree, you could even consider Further Maths, it makes you very favourable for Maths degrees. Furthermore, economics would also help! Economics is also benefical for Law, although no University really requires this. Politics is also great for Law, and will set you up well for the future. They're all good subjects! I'd recommend choosing your third based on what's the most practical, and choose a fourth you find genuinely interesting and are passionate for. The workload requires a passion!
Regarding subjects, do whichever you find the most interesting and think you'll get the best grade in. None of those subjects (Econ, Criminology or Politics) are required for any university degree, so just do the one that'll get you the best grade. It literally doesn't matter what's more connected to a Law degree or what looks better; you mainly just need to focus on getting good grades.
Alternatively, consider doing FM as your third (and final) A level if you think you might want to pursue a Maths degree. In any case, make sure to talk to your school guidance teacher about university degree choices / apprenticeships / acting.