Masters degrees in the UK are one year in duration, as oppose to US where it's two years right? I think you may find the content of a UK masters a little more advanced than a US masters, since in the UK students embarking on a masters course would have completed three years of solid physics (I'm contrasting this with the US bachelors which you will also have to do courses in your minor etc, so not just straight physics....correct me if I'm wrong). Another issue you might find is with funding, there tends to be very little support for students wishing to do an MSc (especially if they're from outside the EU).
Imperial is a world renowned university, it's prob up their with the top ten US places in terms of rankings, or just below. I don't know anything about quantum optics research but from what I've heard the particle physics MSc at Imperial is very very good, which perhaps is a good indicator for other fields.
If you're just doing it to gain research experience to make your PhD application look better, couldn't you just do an REU or whatever theyre called in the US, over summer or something? Might be a lot cheaper and easier......otherwise even if you can secure funding that road would be very long 1year UK MSc, followed by 5-6 year US gradschool, where you'd prob repeat lots of the MSc in your first couple of years wouldn't you?