No. The conditions for an A* are as follows: you need 80 percent across the whole A-level (AS + A2) and 90 percent across just the A2 modules.
You could get an A* even if you had 70 percent in physics. It's mathematically possible to get an A* even with 60 percent.
Here's how it works:
1. (70 percent at AS + 90 percent at A2)/2 = 80 percent average, 90 percent at A2. Grade: A*.
2. (60 percent at AS + 100 percent at A2)/2 = 80 percent average, 90+ percent at A2. Grade: A*.
You may want to resit to increase your chances of getting an A just in case you muck up your A2 exams but from a mathematical standpoint, there's no reason that you can't get an A* with only 75 percent at AS.