Have you done any research at all? You'd either have to do an Economics degree and then go for Graduate Entry Medicine, or get into Cambridge Medicine and then intercalate in Economics. Not sure how possible the latter is, they say you can do a BA from a range of subjects (as opposed to an obligatory biomedical Bsc at other intercalating med schools). Unless you can pay for the ridiculous fees, or you think you're competitive enough for need-blind admission, it's probably not worth thinking about America.
If you're really set on doing both subjects, GEP Medicine is probably the most plausible option. Generally it wouldn't be recommended to set out on this route though as GEP is a lot more competitive than undergraduate entry. You still have time to rethink whether it's worth the risk and money.