Firstly dont get too worried ... you havent even got the results yet so you may have done better than you think. If you only did badly because of your personal circumstances then you are obviously very clever and will hopefully do well in a levels ... a levels are what really counts they are more important than gcses. Also if you explain to them the circumstances im sure they will be understanding. Your interview and personal statement is also important and your UKCAT so if you do well in all those things im sure you will fine.
When you apply to do medicine you apply for four medicine courses and one biomedic degree as a backup as you can get this degree first and then reapply for medicine or just use this degree for a diffrent but related job. Also you it is quite offten that people change there mind on what degree what to do before they apply as they do there research ... you wont be applying for another year but it is good to begin your research now but keep it broad you never know what situation you might be a year from now.