You can find American shops in the UK online. There is one near me in Lacock that since I am so close, about 5 miles, I can just go to it, but it's really a mailorder place. You have to walk through the whole warehouse before you get to where you can buy individual items, and then they tend to be the ones with some spent shelf life.
I've found that just about anything is available someplace...even if it's down to finding a company online that ships overseas. I was just in The Netherlands and found Skippy Superchunk Peanut butter in a grocery store. So I didn't exactly have to go back to the States to get it, though I'm sure it's also sold somewhere in the UK.
Where there's a will, there's a way.