4th AS level failed

I am doing A-Levels internationally, therefore AS levels account for 50% of the grade. I just finished my 12th year and completed 4 AS subjects. My subjects are Maths, Physics, Economics and Computer Science. So, initially, I wasn't planning to carry on with comp.sc as I only wanted to try it out, putting the focus on the main 3 subjects. During the examination period, It was my last priority and i did a very small amount of revision compared to other subjects. I got a C for my mocks, but received a U for the official exam. Should I tryhard and attempt a resit in october or drop it and leave it behind since i dont need it? I am afraid that this grade in my record will ruin my chances at applying to Russell group universities. If I get great grades in my main 3 subjects, will that U stop me from going to top univeristies?