Slightly academic response:
Many people get their 'views' of other cultures from TV. Especially if they have not met people from that culture, or been to the location. (And maybe from books?)
So, that said, American shown in the UK usually consists of a lot of TV sitcoms, Hollywood films, crime programmes, and 'Reality' shows.
Many people in American 'Reality' shows are rich, and beautiful, and many people in sitcoms are middle class, and funny, perhaps a bit stupid, crime shows have people who are smart and tough, and Hollywood films often have 'every day people' who are extremely good looking...
And then there are the American tourists... People all over the world say they are 'fat and loud', although that is statistically impossible... They just don't notice anyone who is not 'an extreme' of some sort.
So, extrapolating from this: Americans are fat/beautiful/rich/stupid/middle class/smart/loud/funny/tough. (Depending on what you watch!)
In the USA, we get a lot of Hugh Grant, and 007, shows about English royalty, and Monty Python. English people are often good looking, well dressed spies, or rich criminals, or romantic men with money, or silly/witty/funny.
So, the generalisation would be: English people are silly/well dressed/funny/criminal/royal/romantic/rich/witty... and able to kill you with a slight of hand move.
I don't really know anyone on 'either side of the pond' who would fit either description.
And I don't think most English people expect all of America to live like, and look like, the Kardashians.
