I’m not sure what point you are making. If a UK student didn’t take or got a poor grade in maths GCSE (say a 6) then they wouldn’t get into Oxford, whatever degree they applied for, absent some very compelling explanation.
If a UK student took a random set of (especially non-core) science and humanities A levels then they likely wouldn’t be a viable candidate for Oxford (my nephews and nieces effectively ruled themselves out by doing A levels like geography and art). I don’t think we know if a top US university would look down on doing something like that, it would certainly require a lot of explanation in the application.