I'm no expert, but I think I can answer a few questions posed above.
From what I understand of the registration process, you must have completed an "internship". In the UK, this appears to be FY1+FY2. If you are an IMG, you must have completed a recognized internship in your home country.
You also have to sit the PLAB unless you can prove that you have had a postgraduate qualification within the last 3 years. What they consider as the postgraduate qualification, I do not know, but I submitted both my USMLE III and Internal Medicine Board, both of which were over 3 years (and I had to provide further documentation that I have been in active employment since).
Furthermore, in order to be registered as an IMG (and you are an IMG if you received your education in a country other than UK/EU) you have to sit the English exam or have your previous employer attest to your English skills. Note: this requirement does not exist for graduates of schools in the EU, none of which (outside the UK) have English as their primary language.
cbcgingko: The GMC only deals with registration of physicians, it does not evaluate a physician's training to determine if they qualify for specialty registration. The PMETB deals with that. Emergency Medicine is a recognized specialty, so there is a separate Royal College for this specialty. The PMETB considers recommendations by the various Royal Colleges in order to determine what is appropriate post-graduate qualification. My guess is that if you complete training in Emergency Medicine and had Board Certification in it, it would probably be accepted. But the best way to find out is check with the Royal College of Emergency Medicine, or whatever it is called.
From what I've read on the GMC website, I'm not sure that registration on the GMC qualifies you to practice elsewhere in the EU. You'd still have to register with the medical councils in the other member states. I know doctors in other EU countries still have to register with the GMC if they want to practice in the UK. Also, I was told that even if my qualifications are accepted by the GMC, it does not mean it will be accepted with all EU states.
JD028: From what I know through a colleague at work, FP training in the UK should be equivalent to GP training in the UK. Therefore, if you apply right after training, the process should be fairly straightforward.
orionarmdoc: Since you trained in the US, you would not be an IMG there. Did you finish training there, or did you leave before finishing? If you haven't finished, you could probably find a training program somewhere to complete it. I don't think two years is all that long. If you have family commitments, I think it would be harder to do the training in the UK.