Ya it is that England are in good position but cannot say about winning with big margin, as 5th day the swing is very low so if South Africa play smartly then they can get close to getting draw this match because anything can happen in cricket so their will be lot of drama on 5th day.
Good four top orders already got out and SA still requires another 375 runs and only 6 wickets remaining. Winning this match will be almost impossible but they can give build good partnerships then they can at least try to draw the match. By looking at their batting performance in this series mostly England is going to win the match but not sure whether with big margin or small.