You only require 3 A-levels to get into medicine, and taking "extra" A-levels isn't going to score you "bonus" points when applying; offers are normally made on the basis of 3 A-levels and those applying with 4 or more A-levels are usually only required to get passes in any additional A-levels offered. There is no benefit in taking extra A-levels to apply to medicine (or indeed, anything else, unless you are taking maths, further maths, and two other A-levels).
As far as IB vs A-levels go, there is no preference as far as admissions go for any particular examination system provided you meet the subject and grade requirements of the course(s) you are applying to. It is possible that one or the other might make things slightly easier once you are on the course, however in the case of medicine the primary barrier to entry is getting onto a medical degree in the first place; after that point, it's more or less a conveyor belt to a foundation post unless you fail something in the course.
may be able to correct me if I'm wrong (and no doubt advise otherwise on the subject), but my general impression is that universities, including medical schools, don't particularly care about extra-curricular activities unrelated to the degree programme in question. For medicine what is "relevant" might be a little more broadly defined than most other courses as demonstrating e.g. leadership ability, team work skills, and empathy are important qualities for prospective doctors. However things like, playing an instrument to a particular grade, doing creative writing, writing some program in code, aren't really relevant except where they demonstrate those abilities specifically.
I think overall you have an extremely inaccurate idea of how medical school (and indeed, university) admissions works in the UK. I would strongly recommend you spend a lot more time researching the entry criteria and shortlisting methodologies used by medical schools, and what they are actually looking for a result. I would also suggest you dispel any preconceptions you may currently hold about university admissions, since they seem mistaken on the whole.