I hope Madden either uses his own Scottish accent or uses one of the English ones he does.
Madden will probably end up doing a English accent though. A lot of people outside of Scotland struggle with Scottish accents. Madden's accent isn't strong enough that it makes him hard to understand but you still had people who watched the Bodyguard in North America confused by accent. There was a whole thing about Madden saying ma'am and people thinking he was saying mom.
Marvel/Disney probably isn't going to spend hundreds of million on a movie with a lead that English speaking audiences can't understand clearly. Far easier for them to just say do the RP accent from Cinderella.
There is the whole Hollywood immortal/ancient world trope of those type of characters talking in a RP accent as well.
I thought Madden's American accent in the movie Bastille Day wasn't great and he is one of those British actors who's line delivery is a bit flat in an American accent. Some actors are so busy concentrating on getting the accent right that their overall performance ends up suffering for it because they are preoccupied with the accent.