Entrance requirements will vary from year to year and in recent times have been driven by the latest changes in the education system. A few years ago, you could score whatever you liked in your A-S levels and, so long as you had good predicted grades, still be in a relatively favourable position to obtain an offer. Nowadays, medical schools will use your A-S levels as a baseline. If you are not doing well now, the chances of you doing well later will seem more remote to them. Some individuals with low A-S level grades may argue that they will work very hard the following year to make up the grades. This attitude will only be met with the question why you are not currently working hard. So be safe - just work hard and get the grades in the first place.