MMC will tell you the answers (or at least, give you new acronyms to be confused about). Google MMC (modernising medical careers).
In short - you graduate from medical school. You do foundation year 1, after which you have full registration with the GMC. Then you do foundation year 2. Try to make sure that you do an A&E job in F2, failing that in F1.
During F2, you apply for ACCS (EM) - acute care common stem (EM theme). This is 2 years of EM, Acute Med, ICU and Anaesthetics. Usually 6/12 of each. During this time you're called an ST or CT1 and 2 (specialty/core training years 1 & 2). Then, at the moment anyway, you have another competitive entry into ST3 onwards, which is runthrough up to consultant grade.
Exams are done during the ST/CT years, although quite a few people recommend taking part A of the MCEM during F2.
So, in summary:
Graduate
F1
F2 with an EM post if possible, possibly part A MCEM
CT1
CT2
ST3 (If I remember correctly, you need to have the full MCEM by the end of ST3)
ST4 onwards. Somewhere you have to sit the FCEM.
College of Emergency Medicine website is reasonably useful also.
No idea how this compares to the US system!
HTH