Your ECs are very strong for a student from the UK. But I’m assuming you feel your GCSEs and A level predictions make you marginal for Oxford?
You mention first gen, but nothing about whether you are from a poor family/disadvantaged community/ethnic minority/state school which would provide a significant boost at these colleges. Clarity on that and whether you are full pay or expect significant need-based aid would help commenters to give advice.
It’s very tough to get admitted for US students and even more so for internationals, so don’t get your hopes up too far. But if I had to bet on whether you’d get into one of these colleges vs into Oxford then I’d say that your profile probably looks better to US colleges.
Having said that, you do need to try and understand the differences between the city and deeply rural environments of a few of these schools. I have a hard time picturing most of the British students I know at Dartmouth.