Graduate entry (GEM) so much more competitive than undergraduate. Be warned, always try to get into med initially!
You can not not apply with out Chemistry, I believe all universities ask for chemistry. Most ask for two sciences, so you can get away without biology.
If you are finding it hard to memerize things at A level, it is only going to get harder staying medicine at Uni, so either try to tackle this issue and stick with chemistry, or is medicine the right option for you?
Looking back at it, I am currently sitting my A2's in chemistry. I would recommend you stick with chemistry. There is a lot to remember (Stupid aqua ion colours in chem 5), but it gets so much easier with you finish A2, and you look back at Chem 1 and 2, they are much more simple, and you can resit.
If you are doing the new spec, it is the same deal, except you don't even have to resit, you get 2 years anyway!