If you dont have an interest in it and your understanding is average at GCSE then you probably wont be able to do ALevel biology. I took it in my first year of A-Levels and while I enjoyed all sciences before (I was in triple science, got A*,A,B in the three) in A Level I couldn't stand it. It isn't the sort of subject you should take for the sake of it being 'Useful'. You should do things you enjoy because at the end of the day its the people who don't enjoy what they're doing who drop out or who fail. Also it depends on what course you want to do in Uni to what subjects are useful or not. If you wanted to do a science in Uni then it's useful, otherwise something else entirely may be helpful. What were you considering?