As far as being unsure whether medicine is for you. I would recommend doing work experience/ relevant volunteering. As well as being required for a medicine application it'll also give you a sense of what medicine might be like. Also it's possible to become a research scientist through medicine.
Why not take a gap year and gain some more work experience?
Hi, I'm an A Level student and I'm interested in medicine but unsure if clinical medicine is a career I want to follow. I am planning to apply for a degree such as biomedical science this coming term and am thinking about then going onto to do graduate medicine afterwards if I'm still interested as I don't want to apply to medicine now when I'm still not 100% sure it's what I want to do. However, I'm worried about the financial cost of this since tuition fees are so high now, and also about the length of studying. How can you fund this, either on the graduate course or the normal 5 years? Is doing a science degree first a bad way to go into medicine?
As other have suggested, it is very unadvisable to take another degree for the purpose you have stated.
If you want to do medicine, do it now. Only do Biomed if you really want to and not because it is a gateway to medicine and yet you are unsure about medicine.
Please note that graduate medicine is more competitive then undergraduate medicine. You have to get a minimum of a 2;1 in your first degree. Then you are competing with a very large number of other graduates for a lot fewer places then in undergraduate medicine.
So - in a nutshell - think carefully what you want to do.