Building on what Carmen said, I'm sure that everyone would be able to notice a difference in the way people from the north of England and the people of the south speak, but the smaller differences between the regions within the north or the south are smaller and harder to notice. For example, take the Queen and a Scottish Highlander (just to go for extremes). They both speak English but there is definitely a difference with their accents.
To answer the original question, Americans sound like they have accents to English people and the stronger accents, when you're not accustomed to hearing them, can be quite difficult to understand. When we lived in Scotland, my mum used to dread picking up the phone because she wouldn't be able to understand the person on the other end of the phone because everyone had really strong accents. Gradually, though, you get used to them and its a lot easier to understand.