For PS, this is either very important indeed and will be scored or considered before interview, or it won't be looked at all. You won't know which it is until you have your strategic choices, which you can't get until after you have done your UCAT (go to the which medical school megathread and post there in the format of the first post when you have it). Those that score it or look at it will be looking for certain things. If you are applying to any of those you will need to carefully work though their application policies and craft your PS to fit. This will take much longer than if you were applying to those that don't look at it at all.
Lots of schools do exams in the first few weeks of the autumn term to refine predicted grades, esp for those that are applying in Jan. If it will make a difference to your predicted grades you will also need to factor that in. If it won't, then again I would be thinking about and talking to your teachers about managing your priorities.