It's a strange (and wonderful) quirk of cinema nowadays that the big films seem to be released first outside of the US. Beginning with Iron Man 2, all of the Marvel films have been released in the UK a week before their American release dates. In the past, when the world was black and white and there was no such thing as the internet, we Brits would have to wait months, or even a year after the American release dates of films to see them in cinemas. The only realistic way we would get to see the films early was if they were on a tiny screen on a long-haul flight.
But now, the trend has reversed. Because of the growth of the internet, the process of releasing films has streamlined. The world has become much smaller due to the way the internet brings people together, which means that distributors are keen to release films as simultaneously as possible. Spoilers appear online VERY quickly nowadays so it is imperative that studios release films quickly to prevent anyone having their experience ruined. Nobody likes a spoiler, and you can only experience something for the first time once, so filmmakers seek to give their audience the best experience possible.
But Why Does The UK Get Marvel's Films First?
The answer came from Marvel Studios oracle, Kevin Feige. As explained on the Marvel Studios subreddit during a Q&A for Avengers: Age of Ultron, Feige says that the reason their films are released overseas first is because many countries have holidays around the end of April (one of the usual release windows for MCU films). Obviously, holidays mean that there will be less people working or at school who need something to occupy their time, this means a bigger audience for the films. Marvel made the decision to release Iron Man 2 and Thor early outside of the US and saw no drop to the box office takings.
In fact, it allowed them to advertise how much money the film had made and how many viewers it had had before its American release. Distributors would feed off the buzz being generated overseas to create anticipation for a film's release. For example, films would be advertised as "the UK box office smash" or "the No. 1 film in Australia," which would also build anticipation. Marvel made the decision initially as an experiment, but as it worked out well — they decided to keep releasing films early outside of America.
Sounds Great, What's The Catch?
Amazingly, there doesn't seem to be any downsides to this for the audience. Marvel simply release their films a week earlier in the UK and allow audiences to enjoy the films before the Americans. It has worked out well, so they continued to do it. But there is one example of when the early release caused a slight disadvantage to the UK audience (though in my opinion, it wasn't a huge problem).
You may recall that after the "Battle of New York" in The Avengers, Tony Stark tells his friends that he is hungry and during the fight he spotted a shawarma place. As it is something he has always wanted to try, he suggests that they go and in a post-credit scene, we see the Avengers enjoying some shawarma in a debris-filled restaurant. This scene was a very late addition to the film, it was actually shot AFTER the LA premiere of the film. During the same Q&A Kevin Feige revealed that all of the cast were put into a van and told that they needed the actors in costume one last time without giving them an explanation. It was only when they arrived at a shawarma place in Beverley Hills that it all became apparent.
They filmed the scene at the restaurant and it was hastily added to the final film. Eagle-eyed viewers will notice that Captain America has his hand across his face during the whole scene, this is to hide the beard Chris Evans had grown in preparation for his next film. Feige had stated the prosthetic they had prepared made Chris Evans look like a burn victim so they chose to have him cover his face the whole time.
Really, a slightly later release allows the American audience to experience the most complete version of a film. However, aside from the one scene added to The Avengers, there isn't really a downside. It's nice that, for a change, UK audiences can see things before they are released in the US. We still have to wait for the TV series to air in America though!