IB is already an enormous time commitment, I frankly can't even see it be possible to do A-levels at the same time - maybe A-level Further Maths, if you're doing HL Maths and Physics in IB, as a single additional A-level. Having done IB, I can assure you that you will be MORE than busy enough with just IB; we had about double the number of contact hours as A-levels students.
If you fail IB but do well in your A-levels (as opposed to the much more likely situation that you fail both) universities are going to be very closely looking at your application to try and figure out what went wrong, why you made such a bizarre choice to overload yourself with work to no avail, and whether you're going to be able to cope with a degree there.
I'd note IB is actually, from what I've heard from friends who went on to do it, quite a good preparation for medicine. You're doing a broad range of subjects, and there's a lot of material to keep on top of, leading you to develop effective strategies to balance a wide ranging and intensive workload effectively. The IBers I know who have gone into medicine have said this has been useful - additionally, unlike most A-level students, you won't have just done sciences and actually know how to write an essay (something a friend of my sisters said specifically was a technique she had to teach her friends in medical school once they started).
At the end of the day though, you should pick, and do, one school leaving qualification - either A-levels or IB. Odds are trying to do both will lead you to just doing badly in both, which suffice it so say will make getting into medicine something you will have to do as a graduate (which has it's own set of challenges) and getting into any degree harder. None of your reasons for doing both make any sense in any frame of reference, so just forget about the "idea" because it's a bad one, regardless of who or what suggested it. Bear in mind your "agency" is going to get paid regardless of whether you fail every single course you take or not, since you've presumably paid up front. Another reason paying to put your education in other peoples hands is a bad idea, generally.
On a side note, I would disregard the suggestion to do IB Visual Art because there is no exam; it is an enormous time sink in an already intensively programmed course, and getting the highest grades (i.e. 7 or 6) is realistically harder than in most other subjects because you can't just "learn the exam". At the time I did IB as I recall, HL Visual Art had the lowest proportion of candidates getting a 7 out of any subject...