Hi, I am not too sure about the new spec introduced in 2015 that you studied as I did the old one (2008 spec) that ended this year. But If resitting exams will help your final grade then I'd do it without hesitation. I know people who got C's and D's in AS and resat the exams they needed to. they came out with A's and B's. I don't think you need to resit all of the exams, maybe just the ones you did really bad on that might have pushed your grade down. you should check your ums in each paper to be clear which ones you need to resit.
Just make sure this upcoming year, there is nothing in the way of your studies. Hopefully you settled well into your new school now.
start properly revising every day well before the exam season, probably around Christmas. I would also make a revision time table because it helped me the most personally.
Your GCSE's are fantastic which is an obvious sign you're well capable of getting the top grades. Also, it is by no means an impossible Aim, if you work your ass off next year, nothing can stop you from getting the best grades.
To answer your other question, unis will not care about your As grades, as long as you meet their overall grade requirements.
I hope this is somewhat reassuring and good luck!