A good combination of A levels would be Business studies, History, English Literature, and Government and Politics. This is only if you are REALLY committed about going down the Law path however. If you want something to back your fall if you don't get the A levels you want or don't get the offers (since Law is a fairly competitive course in the UK) I'd suggest doing something like Biology.
GCSE subjects should be much the same. You do not NEED any specific A-levels or GCSEs to become a Lawyer as Law can be sub divided into many different practices with many different specialisations. It's reccomended you choose GCSEs and A-levels that may allow you to develop an analytical and critical approach to matters. The subjects I listed above can do that, and also develop your skills in writing which is essential to Law. It's all about the personality, and a little bit about the subjects you pick.