It sounds like you are hoping to seamlessly go from medical school in the UK to US residency. Now, that may be possible, but it may not. Firstly, many programs will not even interview you without ECFMG certification which requires you to have graduated medical school and passed step 1, step 2ck and step 2cs. Now there are programs that don't require it, but my experience is that the majority do, and of those that do not, being ECFMG certified will increase your chance of getting an interview - it's getting more competitive for FMGs every year.
Secondly, when will you graduate? Most programs require you to start the middle or end of june. You need to be ECFMG certified in order to start residency. I know graduation for my medical school was July 1, and the diploma was not issued until a month later. You need your medical diploma to be ECFMG certified, and to start work, and to get your medical license (in some states).
Finally, if you are not a US citizen (you haven't made this clear) or dont have PR/GC you will need a visa, ideally an H-1B visa. You cannot get this without having completed medical school, as you need to have passed step 3, which you cannot sit without having graduated. Even on a j-1 it is cutting it fine.
It is rare for medical students to go straight from UK medical school to US residency, and unheard of for british citizens to go straight into a US residency from medical school. That does not mean completion of F1 but if you even want to have the possibility of working in the UK ever again you would need to complete your houseplant days first.
I'm so sorry, I totally forgot to mention the hurdles you have to jump to go straight from final year to a US residency. I've only heard of 2 other people who were able to pull this off, and both of them were American citizens. I'm a US Citizen as well. Splik is absolutely spot on with the previous post.
1) ECFMG Certification - As Splik mentioned, you cannot get this until you have received your diploma and completed Step 1, Step 2 CS and Step 2 CK. Most programs state that you need to have the certification to apply, before the interview or before rank list deadline in order to be ranked. There are very very few programs out there that do not require the ECFMG certification during the application process. But I still applied to programs that required ECFMG certification and hoped that a few of them would make an exception once they saw my app. The expensive gamble/shotgun approach paid off - I received 35 invites out of 115 applications, most of them from programs that were willing to overlook this requirement. I am more than happy to share the list of 35 if you want to. But keep in mind that this is highly specialty, program and candidate dependent. And I am not sure if programs will be as lenient in the coming years as the competition is rising exponentially. I am 100% sure that I would have received many more invites if I had my ECFMG Cert. A lot of doors were closed to me, but I got lucky and still had a good variety of solid programs who were willing to bend their rules a bit.
2) PTAL - This is a letter you need from the State of California to be eligible to apply to programs in Cali. And just like ECFMG Cert. you need to have your diploma to get the PTAL. So being a final year med student, the entire state of Cali shut me out. This is not a deal breaker because the rest of the 49 states don't require this. But I would have loved to apply to a few programs in LA, San Fran and San Diego.
3) Time off for interview - Being a final year student this was extremely tricky. In today's competitive atmosphere I wanted to interview at 12-15 programs to be 100% sure of my chances. Scheduling interviews were a nightmare, most programs don't have the dates you want. Theoretically you can do 5 interviews a week, but this is almost impossible due to lack of free dates, travel between cities, importance of attending pre-interview dinners etc. I interviewed at 15 in total and max I did in one week was 4. I was burned out by the end of that week. I'd say 3/week is a realistic and comfortable number. So you're looking at a minimum of 4-5 weeks off! Keep in mind that most programs don't interview during the 2 week Christmas holiday that we get. I asked my medical school if I could have my 4 weeks of SSA in November and December and use it for interview travel. They said yes and that was a lifesaver!! In addition to that they were also willing to give me an additional 1 or 2 weeks off during rotations if I needed it for additional interviews. Trust me, having a helpful and understanding medical school administration team is a priceless blessing.
4) Visa - I am a US Citizen and didn't have to go through this process. But as Splik mentioned you would be automatically in-eligible for an H1b visa, which is much better than a J1. H1b requires Step 3, which requires a diploma. Even with J1, you would be cutting it close with the UK graduation nightmare which I will get to next.
5) Graduation Date - Most American residency programs start mid-late June and you definitely can't start without an ECFMG Certificate and a State Medical License; both of which require a diploma. My program has orientation from June 11th and proper work starts on June 22nd. Unfortunately my UK graduation date is not until July 11th even though we finish exams end of April and get our marks on May 14th!! As you can see, this is a nightmare.
So I appealed to the Dean and the Head of the Medical College to receive my diploma early. However before it got to the Dean, this process took months and I had to go through so much chain of command and red tape. It was incredibly frustrating. My dean was very understanding and receptive, and approved a special warrant for an early graduation. Even with this, I have to wait until the board of examiners meet and ratify our marks before I get my diploma. June 13th is the magic date! As soon as I get my diploma mailed over on the 13th, I am going to have to run around like a headless chicken and deal with the Pennsylvania State Medical Board and the ECFMG to have everything expedited and done by June 22nd. Who knows if it will happen? Worst case scenario, I need to take a few days or a week of vacation and sit at home till everything gets sorted.
The two other Americans who went straight after medschool, one of them (Manchester) did what I did and got special permission and was able to have everything in time to start work. The other however did not get this special permission from his University (Warwick) and had to use up all 4 of his vacation weeks in June/July until everything was done. I can't imagine working 11 months without a single vacation day.....it must have been brutal! But a lot of programs will let you start late and finish late (3-4 years down the road, depending on how long your residency is) rather than use your vacation time while you wait. I'm not a big fan of this, because it puts you off cycle for fellowship start dates and you'll end up losing a year down the road.
As you can see, there's a lot of pain and suffering that one has to endure to go straight from medschool to residency. Our curriculum and dates are not streamlined for a US residency like the Caribbean schools. It's incredibly difficult and takes a lot of planning, forethought and tons of luck! And you definitely need a kind and understanding medical school administration!!