Well there’s no set requirements. As good as you can manage. The grade offers to do science subjects an university vary quite a bit from As at Cambridge to much lower at other universities. When you start looking at universities and find one you like you can find out their requirements. There are lots of jobs in science and some require better marks than others. If you try your best and are enthusiastic about science and willing to learn you will more than likely be able to study science and get a science-related job 🙂
Being a scientist really just means you ask a lot of questions- ask “what’s that?”. You can be an amateur scientist and simply question the world around you. To be a practicing scientist the requirements are also quite flexible- in my lab we have people w, bachelors, masters, PhDs, MDs and even some high school students working in our lab. All you really need to show is an enthusiasm and curiosity and in my mind you are a qualified scientist!