I’ve just finished by first year of biochem at Imperial. The answer above gets the idea: find some documentaries and books about biochemistry to start off with. Online courses and attending talks and events are also very helpful.
Oxford publishes a “reading list” that can give you some ideas to start off with, but I really enjoyed Genome (Matt Ridley) and Power, Sex and Suicide: Mitochondria and the Meaning of Life (Nick Lane). I’d also recommend you go to your local library or large bookshop and look in their science section for anything relevant that interests you. Don’t just list what you’ve read though, when you write about them you should pick out the interesting points you found in those books (maybe insights into topics, or what spurred you on to find out more about certain topics).
I know that personal statements have changed (mine was one long-form response, I think it’s now broken up) so can’t give you specific information on that, but I can say that your supercurriculars are what you should be writing about mostly.
RS is an interesting third choice of A-level (maths is the most common third A-level done, though lots don’t have it, including me, so don’t panic), so it might be interesting if you find it gives you any interesting insights or perspectives into what you’ve studied in biology and chemistry, or other research you might do. Something like that can help you stand out a bit, and shows a bit of personality (personal statements are personal - they want to see what YOU are like): I did history and french at A level and talked about studying the history of science and how I’d then done a project in french on development of vaccines.
Sorry, this was a very long post, but I hope that at least something in it can be helpful - do ask if you have any other questions about applying (to other unis as well) or about the course!