I'll try to answer some of your questions. Go asap to US. It really doesn't matter much what you've done outside of US. I think your UK background will help you getting a residency position but not a fellowship. If you agree on j1 visa you can get a residency spot at university program which will help but even if you match to a community hospital, it's still possible to do research. Unfortunately nowadays GI programs care only about basic science research so if you are doing in your Phd something related to genoms, proteoms, etc. and can publish in journals with high impact factor it might help but not sure. GI is competitive even for american grads.USMLE scores matter for residency but not much for a fellowship unless you failed or have very low scores. Take the exams asap and apply for a residency next year.
FYI: I am an IMG, did residency at good university program, have 4publications +2in process (all first author), posters at international meetings, one year of hepatology and still no chance for GI, i couldn't match three years in a row....