Some med schools accept retakes (without ExCircs) and some do not, so like everything else, there is no easy answer!
Definitely worth OP resitting Eng Lang, but there will still be a few places that will not accept this. Chem is harder to call, as few people getting A+ at A level will have <6 at GCSE. Again, some places would be OK with the A level alone, some will score the GCSE, so that grade would put them at a disadvantage.
Personally, I would have thought if on track for an A* at A level, then a 6 or better at GCSE resit should not be a tough thing, and I would probably resit both, keeping most options open. However, if it will affect A level grades, then just the Eng Lang resit and write round to ensure who would be OK with a 5 in chem GCSE and an A star at A level.
In terms of specific unis (and not all-inclusive) - Leicester allow English and maths, nil else, and look like they will only score first sitting, even if resat, so essentially, only useful to meet minimum requirements.
QUB only allow resits if the GCSE was originally taken early
Nottingham (and Lincoln), Edinburgh, and Birmingham do not accept GCSE resits. It looks like Aberdeen do not.
Bristol allow 1 resit of any exam at GCSE.
Sheffield allow single resit per GCSE and all resits should be taken in the same sitting)
Cardiff and Aston are OK if done within a year of originals.
Keele will accept but not if applying with pending grades (must be completed).
Imperial, UCL, SGUL, Sheffield, Manchester, HYMS, Edge Hill, UEA, ARU, St Andrews OK with resits.