okay - from my understanding of reading through various pdfs (the ones from UKBA and some parlimentary transcripts available online - yeah... I spent a good deal of time), this is what I take to be true:
a) as it stands, you complete your FYs under Tier 4 - Postgraduate Doctor or Dentist. You apply to renew your visa at the end of your Year 5/6, where you should have already secured a place. The Resident Labour Market Test is waived in this case, as this is more of a academic job, rather than one for personal financial gains. Also, they also have to allow this as only the completion of FY1 grants GMC Registration, which in turn is necessary in many nations to get a valid license. This includes various SEA nations. I am not so certain about ECFMG cert tho.
b) After the successful completion of FYs, you will
most likely not be able to stay. There is no set Visa restrictions per se, but the rule is you have to satisfy the Resident Labour Market Test. This is a test where if there are people with your abilities available who possess EU Citizenship, you cannot be sponsored. You need a sponsor to sponsor a visa, and they need to provide documents showing that they have tried to recruit EU nationals with the same abilities as you, the non-EEA did. It'll be highly unlikely that you'll be able to pass this test, as far as I'm aware, since the UK, being an English speaking nation, has tons of EU docs flocking to it at the moment.
c) Only those who are
currently on their FY1/2 Programs, on a Tier 4 visa may apply to transition to Tier 2 without RLMT being needed. This is due to the promises and immigration statuses made years back, which politically have to be upheld, I believe.
d) The majority of IMGs over in the UK have been there a long time, before the UK opened tons of med schools, and there are enough UK/EU docs to fill up positions currently (a surplus even)
e) In short, you can't specialize in the UK. Or that's as far as I can understand with some conversations with seniors, and docs.
*disclaimer: don't take my words as being the absolute truth tho. If you really wanna be sure, perhaps seek legal counsel from the countless number of lawyers in the UK.