If you intend to practice in the UK longterm you should get into the UK training system sooner than later. GP training in the UK currently follows a 2+3 postgraduate training format (1st two years common to all specialities - final three years GP specific "registrar" or "runthrough" grade).
It is unlikely that US GP training of 3yrs will allow you to practice as a UK GP. You may well however get credit for your US residency and be able to enter a three year UK GP registrar rotation. These are well paid by US fellowship standards - (in region of 80-90K dollars/yr). They are also increasingly hard to get as GP becomes a more attractive option with the new contract (no nights, no weekends, approx 100-120K pounds/yr <175-210 dollars>)
The easier way would be to get a place on the first two years immediately after finishing medical school (called "foundation training" - previously known as "housejobs") then you'd be in a great position to apply for a GP registrar rotation.
The hardest way would be to do a 3yr GP residency in US and then a fellowship to make your training up to 5yrs - then apply to a UK agency called PMETB to have your training experience recognized as equiv to that in UK schemes (a hit and miss option as this process in flux at moment).
If you just want to come for a little visit (1-2yrs) then there are several schemes to bring doctors over on "International fellowships". These bring foriegn trained docs over for a couple of years but don't give any rights to stay long term.
Hope this helps. In the first instance the Royal College of GPs is probably the best place to call followed by the "postgraduate deanery" in the place you want to work.
