Personal opinions aside, British actors are some very versatile actors. And because of that, we have gotten some amazing performances; regardless if it's the biggest blockbuster or the smallest of indie films.
It's almost like saying that to Australian actors, Japanese actors, or hell; American actors.
That being said, Emilia is also someone that is well known; due to Game of Thrones and what not have you. It makes sense that, especially with a story like Han Solo; you get actors and actresses that can ensure great performances overall. I would also say that as of now; they have done well when it comes to casting in roles for Star Wars so-far, so I'm willing to give the benefit of the doubt as long as they do great.