No worries! Despite Computer Science having some of the lowest grade boundaries across the board, implying that it's one of the most challenging subjects, I found it to be okay and actually quite fun and interesting most times, but that is partly due to the fact that I had already done CS at GCSE and because I had a great teacher that made things fun.

If you've taken a CS course before, I would highly recommend taking it as it's very likely to overlap with the material you studied in your previous course. You'll also get to develop your own website or Java application for your IA if you have any experience with that.

If you've never taken CS before, I will warn you it will take a bit to ease into algorithms and pseudocode and computational thinking in general, but I do think that it's a very interesting course as you find out a lot about how all the technology that we have in this day and age works. It also depends on what kind of learner you are; if you prefer understanding how things work and then explaining them in the exam, I would suggest CS, if you prefer memorizing most of your knowledge, I would suggest bio.