In the UK the movie is called "Avengers Assemble"
Why was it given a different name in that market?
The Avengers was given a somewhat different name because of the British TV show The Avengers. The new name was to help keep them separate and to avoid any confusion for audiences in the UK.
In February 2012, Disney announced that the film's title would be changed in the United Kingdom to avoid confusion with the British TV series of the same name, as well as its 1998 film adaptation, but this led to confusion over the film's actual title. Empire magazine reported that the film would be titled Marvel Avengers Assemble while The Hollywood Reporter said that it would be called simply Avengers Assemble.
They changed the name in UK because of having the same name to TV show The Avengers. So they changed the name but only for UK.
“Well sir, with people who associate the name ‘The Avengers’ with the TV show, there’s a slim chance that they’ll get confused by the movie poster and so may not want to watch our movie.”
Avengers Assemble name is selected because The Avengers team say it every time they go into battle in comics and its the most common phrase, So that's the whole reason for this.