peace be upon you Tay,
American films are treated like the mainstream films here in the UK. Most likely very similar to the US. They are marketed significantly (TV, newspapers, magazines, billboards, fast food giveaways etc). Then, there is a minority market for other films (i.e. not as popular). Although British films tend to do best here than in other places, which I guess is what you'd expect.
Picture/sound quality, perhaps they are regarded as better than the UK, but acting wise, it depends on the actor/actress.
Movies tend to be released several months after the US here in the UK. Sometimes quicker if its a big film.
Generally speaking, the UK is very much like the US, just more expensive. There is a significant anti-US sentiment in the UK however, although thats probably amplified due to the current US presidency.
Wakas