I'm not sure about American cheeses in the UK... I've never looked for any because I'm not really a fan of American cheese - too processed for me, I'm afraid (give me a nice crumbly, mature English cheddar any day).
Plus, there are so many other types of cheese to choose from here that I don't even think to look for American cheese (I'm British and one thing I really missed while living in the US was a decent choice of cheeses - my favourite UK and continental cheeses seem to be expensive and hard to find in the US).
I just had a look on the Tesco supermarket website and they stock over 130 cheeses, including 68 different cheddars (all British or Canadian varieties it seems though).