You cannot become a lawyer by studying only criminal law, even if you intend to practise in that field. You have to study constitutional and administrative law, contract law, tort law, equity and trusts, land law, criminal law, and EU law. You do that by taking a law degree which includes those subjects, or by taking a degree in any subject and then a graduate diploma in law (GDL), or nowadays by taking the new solicitors' qualification routes. About half of the lawyers in the UK have degrees in law. The other half have degrees in all sorts of subjects.
You should choose A level subjects which are academically rigorous, and which you think that you are likely to enjoy. You should then choose a degree subject which is academically rigorous and which you think that you are likely to enjoy. I would not recommend A level law - it's too lightweight, and of little use to an undergraduate or a practising lawyer.
Being a lawyer involves a lot of reading, a lot of analysis of written information, and a lot of writing. Please note that the use of correct and precise written English is important for lawyers. Studying subjects such as history, English, politics, and philosophy can assist in developing the skills used by lawyers.
TL/DR? Do what you enjoy and are likely to excel at.
Miss S Byng MA (Oxon), Barrister.