Maths, Spanish and either Economics or Sociology or even History would be an 'ideal' combination of subjects. Especially if you're applying for Law at Oxford, Cambridge, LSE, UCL, KCL, SOAS, QMUL, Durham, Warwick, Bristol, York, Glasgow, Edinburgh, Manchester, Leeds, Newcastle, Southampton and Exeter.
Don't take Law at A-Level, as it is very different at degree level.
A-Level Maths is not difficult. Because you have TLMaths, HindsMaths, BicenMaths and ExamSolutions YouTube channels which produce A* and A grades every year by those who use the videos effectively. 😉 lol