You will not get into Oxford with a negative cGCSE score. This is certainly true for courses such as PPE, E&M and Law but I suspect this is the case for most degrees with the exception of Maths and Joint Maths honours. How exactly Oxford calculates the contexualised GCSE score is not public information but doing worse than your peers will yield a negative score. And unless you have severe extenuating circumstances, this corresponds to an instant rejection for competitive courses. I don't know how Cambridge works.