I think a lot of it just comes down to media. How much attention does America get in the UK media? And then compare that to the other way around. Most Brits know more about American politics than they do about any European country, partly because of a lazy media that can't be bothered with pesky things like translation. A comparison for us though might be how little the average Brit knows about Ireland compared to how much the average Irish person knows about the UK. Every now and again, I look up how to pronounce Taoiseach and every time I read it, I forget, because it's just never mentioned in UK media.
But with America, one that really spelled it out for me was when I was watching a little news feature back in the day about the success of Bob the Builder. It was talking about how many countries it had been exported to and showed a few clips in different languages. And one of them was Bob speaking in an American accent, as if American children couldn't possibly handle a British accent in their cartoons, so they had to dub it. Of course Peppa Pig has ruined that now.