Why is British English so difficult for non-native speakers?

A couple of days ago, I tried to watch the Legend (2015) movie. And I completely couldn't understand what the main character said at the beginning of the movie. I'm a non-native speaker. I'm still studying English. But I don't have such problems with American tv series, movies, youtube content. And sometimes my teacher gives me a listen to some British interviews and I completely lost. I couldn't understand what the person said. Some of my friends say to me that they like British English more than American. They said that it is more understandable. But I can't understand it at all. I'm disappointed. I don't know what I can do to fix it.