Attempting to shed some light on the initial question of eligilbility timing for UK grads:
Certainly, here (UK) once one has the degree they are eligible to provisionally register with the General Medical Council (GMC) (I assume akin to limited licence in the States) and after satisfactorily completing one year, receives full registration. At this point, we are able to apply for SHO posts (more akin to residency, although not quite a speciality-specific initially).
Whether provisional registration (i.e just completing UK med school) means one is eligible in the States, I'm not sure - I would assume so. Having said that, most people would complete their year of house jobs since having left the system it would be much harder to get back in. Additionally, most medical schools have a local 'matching scheme' with various close hopsitals which favours both parties, encouraging posts to be filled and a degree of certainly for the first year. These schemes may not be open to returning grads.
This is changing slightly however with the new foundation year system which is much more similar to the US residency system. I am less clear on this as it is relatively new and hasn't affected myself or my colleagues. One would have two years of initial training in various generic specialities and then would immiediately apply for a specific speciality-based rotation. Currently, UK junior docs finish house jobs and traditionally do a six month ED job before doing perhaps one or two more 'stand-alone' posts and then applying for an SHO rotation which takes you up to around the time that you're expected to get your chosen post-graduate exam (MRCP, MRCS, FRCA etc).
I digress.... bascially the question is whether provisional or full GMC registration is needed - I would intuitively think provisional only.... but am unsure since I qualified some years ago and the question never arose in my case.... I, and many others, would warn against leaving 'prematurely' since although the thought of return improbable now, it may prove more difficult to come back easily to house jobs, especially those within teaching hospitals.
However, if completely sure that a UK return is out of the question, crack on asap, especially in view that pretty much regardless of length of training in the UK, everyone has to go back and start at the intern level since little or no credit will be given for overseas training (some caveats do exist)...
perhaps that helps, perhaps not...