No particular advantage for applying with achieved vs predicted grades as far as I am aware.
If they meet the standard entry criteria then they will be competitive for it. A-level/IB grades are largely a tick box exercise for medical schools - they check all applicants to see if they meet the minimum academic requirements, and then after ticking that box will normally score students on other factors depending on their specific methodology (which may be UCAT, BMAT, contextualised GCSE scores, or some combination).
Note that all medical schools have their own shortlisting methodologies and just because someone is a strong applicant for one doesn't necessarily imply they will be a strong applicant for another. One medical school may place a lot of emphasis on GCSEs, which may work to an applicants favour or not depending on their results, while another may not score GCSEs at all (which is good for those with weaker GCSEs, but those with strong GCSEs can't leverage that strength when applying there).
Medicine is all about applying tactically - to do so applicants need to actually understand the process of how medical schools shortlist and select students, and all UK medical schools are extremely transparent about this and publish a wealth of information explaining their processes on their websites. Also I'd note that the newer medical schools like Kent and ARU are usually statistically the most competitive.