Short answer: You can't.
Long answer: You need A-Levels to do Medicine I think and it's very competitive, I doubt any people with BTEC get accepted as the highest grade in BTEC is equivalent to A in A-Levels and I don't think many Unis accept people without A* for medicine. A-levels are harder than BTEC's and also in BTEC they don't expect you to memorise meanwhile in A-Levels you need to be able to retain information. If you want to do medicine bur aren't good enough for A-levels then you aren't good enough for Medicine since medical school is very hard.
What you could do is: Apply for a biomedical degree and swap. But for you to swap you'd need to pretty much be one of the best in your year and i doubt people with BTEC can pull that off since the gap between University and BTEC is quite big.
I'm not very knowledgeable about Biology since I'm doing Physics at A-Level but I do know my fair bit about University application and honestly doing Medicine with a BTEC without getting a Nobel Prize or having an insane amount of work experience is impossible.