Usually A-level grades just form part of minimum academic criteria, they want to ensure everyone on the course has a minimum level of ability, but they usually use other pieces of information to decide between applicants when shortlisting or making offers. Note that UCAT cutoffs may only be for invitation to interview, they may still consider UCAT score after interview to determine if they will make an offer. Access schemes are usually for students who meet specified widening participation criteria e.g. being care experienced, being from deprived regions, first in family to go to university, etc.
Also bear in mind every medical school has it's own selection criteria - Manchester having a cut off of 2750 vs Leeds having 2900 doesn't really make any difference except in guiding your choice of which medical schools to apply to. As medicine is about applying tactically, since all medical schools in the UK are exactly equal in the eyes of the GMC and of the NHS (the latter of which is the only provider of graduate medical training posts in the UK and so all medical graduates remaining in the UK will work for the NHS).
Medical schools are extremely transparent in what their selection criteria are. You need to not be thinking about "good" or "average" when it comes to medical schools (since as noted this is not something that exists in the realm of medicine) and instead look at your profile as an applicant, what areas you are strong in (e.g. A-level predicted grades; GCSE achieved grades; UCAT score; etc) and find the medical schools whose selection criteria you would score the best against to maximise your chance of getting an interview.