The UCAS scoring system, for admission to any UK university, is based entirely upon post-GCSE qualifications (such as A-levels or the International Baccalaureate). GCSE results don't count for UCAS scoring purposes:
http://www.ucas.com/candq/tariff/index.html
However, that doesn't mean that university admissions panels don't take GCSEs into account. Oxford's law school states "most of those admitted will either hold or be predicted to obtain all As at A-level (or equivalent), and will already have an outstanding group of GCSEs (or equivalent) dominated by A*s and As". It's unlikely that they will differentiate between candidates who have 8, 9 or 10 GCSEs as long as there is clear evidence that the candidate is of the required academic standard:
http://www.competition-law.ox.ac.uk/undergradu ate/admitfaqs.shtml
and
http://www.admissions.ox.ac.uk/courses/enreq.s html
At Cambridge, admission standards are set by individual colleges (and not by the Faculty of Law) but the same principles are likely to apply.
You should be aware that some independent schools tend to push their students towards Oxford or Cambridge simply because this tends to impress (at least some of) the pupils' parents. Don't limit your choices to these two universities. A law degree from the University of London, for example, is at least as good (and possibly better) than an Oxbridge law degree.
Chris