Do stars fake their British accents for the fun of it?

I've noticed that stars like Madonna and Johnny Depp suddenly seem to have British accents out of nowhere now that they're older and it seems really weird to me. My friend says that they do it for the fun of it and simply fake their accents while my other friend says that these stars have spent so much time in the environment of the British people that they picked up these accents. I wanted to know what you guys thought was the cause of this.