If you want a B, you would need to score approximately 75% or above in Physics and 73% or above in Mathematics. Now, if all you really want is a B then you need not resit, necessarily. Work really hard and you can bump your marks by 6-10% easily. However, like Romeesa said, why not aim for higher? Make your target high so that if it misses, you score just as well.
If you want more than a B and believe you can go that high, then do resit in this exam. Otherwise, work hard for an A* in your A2's; though you can never get an A* with your grades, you can get an A.