Ecosse_14 has the right idea. If you are hoping for As and Bs at A level, choose subjects that you have a real interest in - you are much more likely to do well in your A levels if you do.
The Russell Group has published a list of 'facilitating subjects' which are the subjects which are most likely to be required in general by Russell Group universities: Maths, Further Maths, Biology, Chemistry, Physics, Geography, History and Languages.
It may be worth having a think about where you might want to study law and seeing what subjects they recommend or require. For instance, Edinburgh's
minimum requirements for undergraduate entry are:
"A Levels: BBB including English Literature or English Language. GCSEs: Mathematics or an approved science at Grade C. Note: English Language and English Literature GCSE both at Grade A are accepted in place of A Level English."
You don't want to get half-way through another round of A levels and find you don't meet some of a particular university's requirements! It might be a good idea to contact some universities in which you have an interest now and ask them what they would advise. Good luck, whatever you decide to take.
Naveed