What would you plan to study if you did go to the US? In my opinion if you were just going to then study a premed curriculum and be a bio major and do medicine anyway, it seems like you're just delaying your original plans for little reason or benefit. Unless you had a fairly clear idea of what you wanted to study, and it's something you probably won't be able to pursue again in future and the experience will be somewhat unique to where you're going in some respects (e.g. ancient near eastern studies at UChicago, physics at CalTech, linguistics at MIT, philosophy at NYU etc) it seems a bit pointless.
Note also that pursuing medicine as a second degree in the UK (either standard entry medicine or graduate entry medicine) will have more limited funding than studying it as a first degree. GEM courses you need to pay ~2/3rds of the tuition fee yourself for first year, while getting 1/3rd paid through tuition fee loans from SFE and getting a maintenance loan from SFE, and thereafter get full funding from the NHS. Standard entry medicine as a graduate is even worse (but is less competitive than GEM, even for graduates, although the gap is smaller than for school leavers I understand), as you only get a maintenance loan for the first 4 years and have to pay the tuition fees yourself (probably using the maintenance loan, requiring you figure out how to support yourself otherwise), and only get fully funded by the NHS for the last two years. So there are still some financial considerations if you go the US route.
Also bear in mind there are aspects to living in the US you will need to put up with which don't exist here. You will need to get health insurance, if you get sick there as an undergrad you may have to pay a lot of money, depending what state you are based in and the nature of the student body, you may need to put up with seeing students carrying guns around campus. Also normally in US dorms you share a room with someone, with two beds to a room (i.e. not sharing a flat with individual rooms with others) which can present difficulties if you don't get along with the other person. The drinking age in most states is 21 (not 18 like here) as well, if it matters. Some colleges have arbitrarily draconian rules about some things, and you may feel more like an older school student than an independent adult in uni at some colleges (e.g. one college a couple years ago notably banned dildos on campus, while permitting open carry). Also the nature of assessment is very different; less exam based, with much more of your grade coming from attendance, "participation", weekly assignments and more frequent, smaller quizzes, etc.
If it truly is a once in a lifetime opportunity to pursue something that isn't likely going to be a career goal for you but which has been a long term interest for you which pursuing medicine would mean you would have very limited opportunity to continue pursuing, then it might be worthwhile. If it's just going through the motions to end up in medicine anyway, you may as well just go directly into medicine in my opinion. You could ask both if you could defer entry for a year to spend some more time reflecting on the choices and figuring out what you really want to do, although there is no guarantee either will accept the request.