I've no objection with people changing their mind once they are at medical school - it doesn't have to be a lifetime sentence for everyone that accepts a place. But I do think that there is something wrong about lying to get a place with the inevitable consequence of wasting everyone's time, harming patients, and depriving the world of another doctor. That being said, with a "hate the game not the players" attitude, it's quite likely that you're better suited to finance than medicine.
To the OP: why human biology if interested in investment banking? Medicine is longer, harder, and less transferable than most other degrees. If you have the grades for medicine, why not aim at law, PPE, or something similar at Oxford or Cambridge...? I would have thought they were less hassle, more relevant, and at least as prestigious...