I was in your shoes back in 2011. I put down Neuroscience as by back up, ended up getting 4 med rejections (because of grades) but got into Neuroscience at King's. Ended up going and it was the best decision I could have made! Neuroscience is not only extremely relevant and THE field to be in at the moment but also helps you gear towards applying to graduate medicine if you decide that you definitely want to be a medic (as did I and have applied to graduate medicine this year). The KCL course also has tons of opportunity for research with summer projects in your second year and then a choice of lab research, clinical neuropsychology research or a library project for your final year dissertation.
I missed my offer for KCL but still got in (although that was a few years back so maybe apply to other unis if you're worried you might mess up your exams and get lower than a B in any of your A2s). I didn't do amazingly at a level (ABBBbc) so if you're thinking of applying to graduate medicine I would advise a degree in any biomedical science at a good institution. However, if you have other interests, graduate entry medicine is open to all degrees now at some universities (but remember this means more entrance tests - ie, UKCAT and GAMSAT which is worse than BMAT).
Basically what I'm trying to say is follow your interests as there's ALWAYS a way into medicine if you decide to do it later. Most biomed courses don't even look at your personal statement and will give you an offer on grades alone (again, this may have changed since 2011).
Hope this helps, sorry for the essay.