1) There was nothing much wrong with Brando's English accent- it was acceptable if imperfect. Same goes for Russel Crowe in "Gladiator".
2) For comparison, Eva Green is a French actress who can play an English role with a perfectly acceptable accent. Perhaps it is not Marion's fault that she lacks the skill to do likewise, but that probably means she should stick to playing French characters.
3) There may well be lots of American actors who are incompetent at changing their accent, but there are a lot of British actors who are extremely good at it. Think of Dominic West and Idris Elba in The Wire, Hugh Laurie in House, or the entire British cast of Nolan's Batman movies. I think it is just complacency that disinclines some actors to bother.
I'm afraid that you are so obviously wrong that your opinion isn't valid in this instance.
Have you ever met a British person, or heard one speak?