Hi,
Just to put my two cents in:
I am a UK undergrad, who applied to both UK and US PhD programs (eventually deciding for a UK Uni) and was successful in both applications so I think I know what they are looking for, and there is a bit of a difference.
When you say MSc, are you planning on doing an undergrad masters, MSci, or the post-graduate taught masters, MSc? I did the undergrad MSci and applied for PhDs during my final year - around Nov/Dec for USA and Jan/Feb for UK. I would say that if you are set on doing the US PhD route then the masters may be a waste of time in the sense that you will still have to sit the upper level physics courses in grad school, and won't be able to avoid them even if you have already taken the same material (well, I wasn't allowed to at any rate, and even if you were, the comps/quals will be based on the material taught). So, by doing a masters in the UK you are effectively adding on an extra year to your education (which will be a 4/5/6 year commitment in the US). I decided to do the MSci anyway, already knowing that I planned to apply to the US, as the MSci is a better qualification to apply to UK Unis with and I wasn't sure where I wanted to go in the end (not sure about the MSc, but I imagine its similar). That said, I think the MSci did help me with my US apps anyway, since they saw that I was already taking grad level classes, and would have a good grounding for doing their versions of the course.
The other thing you have to consider is timing - if you plan to apply for the 2013 semester for the US, then start looking into studying for the Physics GRE and sitting it by Oct 2013 at the latest, or April 2013 if you are ready. I was daft and didn't think it would be a popular exam in the UK so when I went to book it it was fully booked for the Oct and Nov 2011 dates (there are only dates in April, Oct, and Nov). So I wasn't able to sit the physics GRE (I did the general GRE after I had applied and then sent the results on to the Unis). Now this is a major disadvantage and pretty much ruled out every US uni as a LOT of credence is given to the physics GRE. Luckily, I had contacts at the two Unis that I was most interested in working at and they kindly waived the need for the PGRE or told me to sit it after I got there. I don't know what the PGRE is like so maybe a US student can fill you in on that.
I think that the US schools pay close attention to the GRE and letters of reference, with research experience and grades after that... at least that was the impression I got. The UK unis I applied for were more keen on speaking about past research experience I had and why I was specifically interested in that field (as well as grades, obv). If I'm honest, the UK application process seemed 'friendlier' for want of a better word and I found the US process more involved but I think this is because they get *so* many applicants that it becomes almost an industrial scale process. That shouldn't put you off at all(!), I really encourage you to apply to the US - just be prepared to spend a good amount of time crafting a good personal statement for the US schools to set you apart, and finding good referees.
Last thing (as this is becoming an essay!) is that in both countries, you should get in touch with Profs/researchers - just an email with CV attached explaining why you are interested in their research and maybe asking some questions about what they see future students working on. A personal touch like this shows you have a specific interest and will hopefully make your name stick out on a stack of applicants.
So, the overriding message from my ramblings is study for and do the Physics GRE if you are going to apply to US Schools.
If you have any specific questions about the application process from a UK student perspective then feel free to PM me :)