It depends, would you be using your back up as a route to graduate medicine further down the line, or just as a separate degree which you intend to follow for the rest of your life? Natural Sciences is obviously a hugely demanding course and would look good on a medical application later in life, but biomedical science is also a very standard graduate-applicant course and would also be fine. In terms of career prospects I would historically say that biomedical science is a degree that leads to a set route of progression career-wise, and that if you get a good biomedical science degree you could go down the lines, very easily, of becoming a biomedical scientist in industry. Natural science tends to be for very clever people like yourself who have wide-ranging interest in science as a whole, as opposed to a specific route -- though I know you do specialise later on.
TL;DR: If you're looking to study medicine and that is it, I'd say stick biomedical science down as your 5th option. Admissions tutors for biomedical science are very used to medical personal statements. If you're into just doing something else, I'd say it's also an equally good option there too.