marvel cinematic universe - Why was the Avengers movie called something different in the UK?

They changeg the name in UK because of having the same name to TV show The Avengers. So they changed the name but only for UK.

They said-

“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 comman phrase, So thats the whole reason for this.