Each medical school uses a different shortlisting methodology. Some score GCSEs and put a lot of weight on them; others don't score GCSEs at all if you meet minimum requirements (usually in maths and ENglish language, sometimes science as well). Of those that score GCSEs, some score all, some look at number and proportion of A* equivalent (8/9) grades, others score just a certain number, of those that score a certain number they may have different weightings for different subjects. You need to actually research the different medical schools you wish to apply to and see what their shortlisting methodologies are. In short you would be best off aiming for those that just require you meet minimum GCSE grades and then don't score them (e.g. UCL, Imperial) and avoid those which put a lot of weight on GCSEs and formally shortlist using those (e.g. Oxford, Birmingham).
Also there is no such thing as a "bad" medical school in the UK - the GMC considers all accredited medical schools equal, and the NHS which is the only provider of postgraduate training positions for medics likewise takes this stance. The NHS goes so far as to blind recruiters from your medical school to ensure there is no bias in selecting for specialty training posts. So your medical school will not directly have any influence on your medical career in the UK - therefore all medical schools are effectively equal and the choosing between them will be based on applying strategically and also considering differences in how the courses are structured and taught (e.g. PBL vs lecture based formats) and factors like cost, location etc.