You obviously know that you will have to be enthusiastic about science and that you need to do well enough at A levels, and go to uni, and study reasonably hard, and try to get some hands-on experience (like lab internships) and I assume you don’t have a problem with that.
I think the most important thing to realise in science is that there isn’t a scouting system like in football. If you want a job (or an internship, or a fellowship or whatever) you have to go and ask for it. Even if you don’t think you stand much of a chance, ask anyway. A slight chance is better than not asking at all. It sounds trivial, but believe me, it’s not! If you keep that in mind, you’ll be fine! Good luck 🙂