I might be wrong but I swear for an A* you need 80% overall of both AS and A2 combined, and 90% in the A2 units. Since you already have 80% in AS this means there would be no point retaking. Of course I may be completely wrong considoring I don't even know how UMS works, so feel free to correct me.
Even if you did decide a retake is best, since this would only be the second time you sat the paper your best mark you got for it would be the one counted, so it can only go up.