I'm by no means an expert, so this is mostly conjecture and hearsay on my part.
I know that for my course, (BSc in Diagnostic radiography) there's no real American equivalent (x-ray techs aren't trained to the same level) but if I was to move to the US, the state I moved to would want me to take their own test to certify me to be able to work there. I am under the impression that I would be allowed to take an accelerated course/ jump right to the test though, as I've already proven competence by getting the certification over here. Of course, I have no intention of moving to the US, so I've paid very little attention to it.
I think you'd have to take a test or two, but I imagine there must be an easy transfer programme out there if you have a look around, since every country wants more doctors, and British medical degrees are seen as pretty great internationally, as are medical degrees from nearly all countries in the world. The American system is a fair bit different though, they have strange limits on doctors because of residency and things like that, I believe it's a pain to specialise in some things due to limits on the numbers of residents.
I'd advise finding a doctor who has actually transferred, and asking him. AFAIK, this place is fairly strict about referring to other sites, but I'm pretty sure you'll have heard of a certain site with a 6 letter name that has a self moderated medicine community in a subforum. The place might best be represented by a robot. Having a look there might find a doctor who's done so.