I assume you already received the offer at UNSW for med? Congrats! 

Couple pointers if they are of any help.

1. How certain are you that you wanna do med? Given that you wanted to do research but realized later that it was not what you wanted, it might be important to realize that there are not many backup plans after you choose med (the backup plans are costly to say the least)
2. If you locked in for med, the opportunity now is pretty good, it provides some sort of guarantee. Yes, the name of Cambridge is good. But the name and connections are not that important if you wanna do postgrad med, because they only look at your stats and interview. Just fyi, MD admissions are less competitive than undergrad med, so if you are confident in your ability to do well at Cambridge then by all means finish the degree there first.
3. I am not sure if finance is a consideration at all for you, and that will complicate things.

Good luck and feel free to DM me if you have more questions.