Remarking probably won't make much difference to your overall grade if unit 5 was a D & you are 43 UMS off an A overall. Remarking usually only makes a difference of a few UMS and you risk it going down as well.
I think the only way you have a chance of improving your grade is by resitting, although you could keep this in reserve to do in case remarking unit 5 produces no improvement; if you're planning to resit anyway then it's not a big risk.
If you do resit check first with the resit policies of med schools, as not many accept resits or a 3rd year to complete A-levels.