There is limited reciprocity between US and UK porstgraduate training programs. Basically you need to train in the country you wish to practice in.
UK postgraduate education is more humane, the hours are more reasonable (50-60/wk on average), the pay is much better (60-80k/yr for interns), but it takes about 2yrs longer to reach to specialist in any field. Once trained you'd be eligible for a year knocked off your US residency (ie: 2yrs for internal medicine) at most - but could go across as a US faculty member but that limits your practice to one academic center.
US postgraduate education as you know is not reasonable hours or salary. It is also not recognized by the UK. There are some opportunities for fully trained attendings in some areas (esp. radiology) to come across to practice on a temporary permit (either on a training permit in big places, or a practice permit in far flung places) but you couldn't turn these into a permanent position. While US medical resources are respected, in general US residency training isn't respected in the UK.
I say all this as a UK graduate leaving the UK to do a US residency.
The other factor is that UK training is close to impossible to get these days (22000 places for 30000 UK grads in this years match). All in all you'd be better off in the US (do derm and moon-light 10hrs/wk - that will recreate the UK residency experience for you!).