as far as I know, most G5 unis care about first-attempt GCSE results in
math and english, especially Oxford and LSE, as these results can determine your essential linguistic and mathematical ability which is emphasized by any degree. medicine, in particular, is
always one of the most competitive degrees ever in G5 unis, hence you need
exemplary grades (in both A-levels and BMAT/MCAT) and a good personal statement to stand out from the crowd. as an international student, you are even more disadvantaged as you'll be competing with talents worldwide for only a few quotas. however, note that your A-level predictions, personal statement and interview performance are
more important, so try to aim for a high grade in A-levels to give prestigious med schools a decent shot! best of luck!
fyi an alumnus from my school (international) received 10 9s in GCSE and 45 predicted in IB, was a medallist in numerous competitions like science and math olympiads, and did a shadowing programme guided by a famous professor from a prestigious uni. unfortunately, he was rejected by Cambridge and he's now reading medicine at UCL instead. international applications to UK medical schools are always cruel, sadly.