Teaching yourself A Level Chemistry will be very difficult, and thats without even considering that you are thinking of taking up 5 A levels. I would not recommend this as a viable course of action.
If you have your heart absolutely set on Marine Biology, and are aiming for a top university, you should try to focus your A levels on the science side to be a more competitive applicant. Your scholarship complicates things, but probably also highlights where your strengths are, are also good at and interested science? You have to be if you want to do a highly scientific degree. If you are, then I would recommend talking to a career/subject advisor if such a thing exists at your school? Or just any teacher with authority that can tell you how your scholarship might be affected if you change your subjects.
Also, I just want to highlight how difficult it would be to get a job in Marine Biology after a degree in case you aren't yet aware. There are so few jobs and way too many graduates to fill them. If this is absolutely your dream, make sure you get work experience in university and really go all out, because you won't find a way into the field if you don't. Just have your expectations right, doing a marine biology degree in no way guarantees that you'll spend your life swimming in the sea studying marine life, it is an extremely small field.