I unterstand that you have narrowly missed a first on prelims, so gained a high 2.1, in spite of the social and acedemic culture shock. I think thats phenomenal. Note also that a first isnt the equivalent of an A, and measured praise mixed with constructive criticism is about as much unbridled enthusiasm as you can ecpect from a tutor there ( see culture shock, academic).
If you were to transfer to the US as a Junior, you may have to scramble to make up core or gen eds in classes with lots of freshmen just out of high school, at the expense of upper level classes with topics that interest and excite you and researchers whose notice you want to gain. And the two years in Oxford with no qualifications to show for may look and feel to you and others (grad schools and employers, for instance) largely like a mistake to overcome, but three years with a degree out of it, a 2.1 or maybe even a first, will look and feel like a success, even though you will remember frustrations and setbacks. You will have persevered instead of given up.
I agree it shouldn’t stop you from checking out transfer options, but I would really urge you to give it another go, and be determined about it.