I think this is the wrong forum to be asking such specific questions. Almost no one here knows how UK medical schools work. I lived in London for two years and have had several family members go through UK medical schools. Unfortunately, my knowledge is very limited but here's what I know:
Getting into medical school in the UK is incredibly competitive. Ask yourself if you'd get into Harvard if you applied. Would you? No? Then you won't get into the Oxford (and other top such as Cambridge, UCL, ICL or King's) medical schools either. Students literally have to get A*s (2400 SAT) scores to get into any medical school in the UK. Furthermore, Oxford medical school admitted 2 international applicants of the 238 applied. That's about 1%. I believe US medical schools (and Harvard undergrad) all hover around 5-10%.
Furthermore, you save NO TIME. Their residencies are at a minimum 1 year longer, and many people take an extra few years to get a PhD in London because getting a consultant (attending) position is very competitive (if you stick to London anyway).
Plus, what are you in such a rush for? Enjoy college and take a few extra classes. If I hadn't I would have applied MD-only vs. MD/PhD. I think you will find you learn a lot about yourself by taking classes you didn't think you'd like.
Finally to everyone saying that the prestige of medical school doesn't matter, I met many Oxford doctors in London who did part of their training at top 10 US medical schools (UCSF and Hopkins come to mind). Rumor has it that Hopkins saves a residency spot for the top Oxford graduate every year. But everyone else is right in saying that you have a better chance at getting a top US residency by attending a top US medical school.