The system to study MBBS in USA and to study MBBS in UK are completely different. At least UK system has some similarities as per PLAB standards but MBBS in USA, the system developed by USMLE - 1/2/3 is absolutely different. The students passing MBBS in India also do not find it easy to give the first step of USMLE. Indian doctors are not easily accepted in UK unless they have gathered 3-5 years of experience before applying for PLAB based entrance test for PG program.
There are two ways to pursue medical program in USA such as giving MCAT in USA or start the journey at bachelor level with the course in a USMLE approved university and passing USMLE 1 Exam after completion of BS at various universities at Caribbean Islands.