If (and it's a big if; you haven't even applied yet!) you don't get in, I wouldn't go for graduate entry straight away. I really don't think it's worth it after being rejected once at the age of 19 (unless the reasons you were rejected are going to take several years to sort out) - it's very competitive and involves spending three plus years of your life studying a subject that at best was only your second choice, when you can very easily take a gap year and reapply.
Having said that, I did put down backups though (NatSci at Bath and Durham, but like AEH says, I'm not you so you need to pick something you like the sound of), partly because I felt safer using up all my choices, and partly as an insurance in case I had a last minute panic about medicine. In reality though, in that situation, I think I would have reapplied, using all six slots for Biology/NatSci and applying to Cam's NatSci course* (which I couldn't have done along with my choices of medical school).
Having gone to open day talks about biomedical sciences, and reading prospectuses, I would avoid it as a subject, as it seems to be essentially 'pre-clinical sciences', with a huge proportion of people applying for medicine afterwards, rather than studying the subject because they enjoy it in itself.
*But shush, don't tell anyone I considered being 'one of them'...