I'd resit the units that contribute the most to your overall grade - the entire year is abit of overkill. Plus by the time you resit them you will be better at the required skills/needs etc. of the top grades.
But i wouldn't worry about it too much mate. I got BCD in my AS, and i needed ABB to get into Uni (like you). I ended up turning it around and walked out with A*BB. So just work hard, prioritise it and focus completely on getting the holy grail (University, haha). Don't let it get you down, and good luck!